/MRKEIEY 

LIBRARY 

UNIVERSITY  OF 
^CALIFORNIA 

EAKTH 

SCIENCES 

LIBRARY 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

GEOLOGY  LIBRARY 
IN  MEMORY  OF 

PROFESSOR 
GEORGE  D.  LOUDERBACK 

1874-1957 


KOCKS,  KOCK-WEATHEKING,  AND  SOILS 


A   TREATISE 


ON 


ROCKS,    ROCK-WEATHERING 
AND   SOILS 


BY 


GEORGE   P.    MERRILL 

CURATOR  OF  GEOLOGY  IN  THE  UNITED  STATES  NATIONAL  MUSEUM,  AND  PROFESSOR 
OF  GEOLOGY  IN  THE  CORCORAN  SCIENTIFIC  SCHOOL  AND  GRADUATE 

SCHOOL  OF  COLUMBIAN  UNIVERSITY,  WASHINGTON,  D.C. 
AUTHOR  OF  "STONES  FOR  BUILDING  AND  DECORATION,"  ETC. 


gork 
THE   MACMILLAN   COMPANY 

LONDON:  MACMILLAN  &  CO.,  LTD. 

1897 

All  rights  reserved 


SCIENCES  Lit, 


COPYEIGHT,    1897, 

BY  THE  MACMILLAN  COMPANY. 


Norbjooti 

J.  S.  Cushing  &  Co.  —  Berwick  &  Smith 
Norwood  Mass.  U.S.A. 


' '  THE  rmns  of  an  older  vtovld  are  visible  in  the  present  structure 
of  our  planet ;  and  the  strata  which  now  compose  our  continents 
have  been  once  beneath  the  sea,  and  were  formed  out  of  the  waste 
of  pre-existing  continents.  The  same  forces  are  still  destroying, 
by  chemical  decomposition  or  mechanical  violence,  even  the  hard- 
est rocks,  and  transporting  the  materials  to  the  sea,  where  they 
are  spread  out,  and  form  strata  analogous  to  those  of  more  ancient 
date."  —  HUTTON. 


978 


PREFATORY   NOTE 

IN  the  work  here  presented  the  writer  has  endeavored  to 
bring  together  in  systematic  form  the  results  of  several  years' 
study  of  the  phenomena  attendant  upon  rock  degeneration 
and  soil  formation.  Although  beginning  with  a  discussion 
of  rocks  and  rock-forming  minerals,  the  work  must  be  con- 
sidered in  no  sense  a  petrology  as  this  word  is  commonly 
used.  What  is  here  given  relative  to  the  origin,  structure, 
and  composition  of  rock  masses  is  regarded  as  an  essential 
introduction  to  the  chapters  on  rock-weathering.  The  por- 
tion dealing  with  the  structure  and  composition  of  the  result- 
ant materials  is  an  essential  corollary  to  these  same  chapters. 

It  is  believed  that  no  apology  is  necessary  even  in  this  day 
of  many  books  for  bringing  out  the  present  work.  The  origin, 
structure,  and  mineral  composition  of  rocks,  particularly  the 
eruptive  varieties,  are  matters  which  have  of  late  received  much 
attention.  In  fact,  it  is  to  these  rocks  that  the  petrologists 
have  devoted  their  best  efforts.  Since  the  introduction  of  the 
microscope  into  petrographic  work,  there  has,  however,  been 
very  little  time  devoted  to  the  study  of  rocks  in  a  weathered 
condition.  The  chemists  have  made  analyses,  but  have  disre- 
garded the  physical  and  mineralogical  nature  of  the  material 
analyzed.  Other  workers  have  studied  the  physical  properties 
of  rocks  decayed,  —  in  the  form  of  soils,  —  but  have  in  their 
turn  disregarded  their  mineral  and  chemical  nature.  The 
writer  has  aimed  to  bring  together  here  such  results  obtained 
by  these  workers  in  divers  fields  as  it  is  believed  will  be  for 
the  mutual  benefit  of  all  concerned.  The  state  of  comminu- 
tion reached  by  rocks  during  the  processes  of  long-continued, 

vii 


yiii  PREFATORY  NOTE 

secular  decay,  and  the  amount  of  leaching  such  have  under- 
gone, are  certainly  of  as  much  practical  interest  to  the  agri- 
culturist as  of  theoretical  interest  to  the  geologist. 

To  the  one,  these  residues  are  essential  to  the  life  and  well- 
being  of  man  through  furnishing  the  soils  from  whence  is 
derived  directly  and  indirectly  the  food  for  life's  sustenance  ; 
to  the  other  they  are  but  transitory  phases  in  the  earth's  his- 
tory, ijepresenting  the  materials  from  which,  through  a  process 
of  fractional  separation  by  running  waters,  have  been  made 
up  the  thousands  of  feet  of  secondary  rocks  which  to-day 
occupy  so  large  a  portion  of  its  surface. 

The  very  general  scheme  of  classification  adopted  in  the 
treatment  of  the  unconsolidated  clastic  materials  may  at  first 
seem  disappointing.  It  has,  however,  been  the  writer's  special 
aim  to  introduce  into  this  preliminary  volume  as  few  new  terms 
as  possible,  using  only  those  which  through  years  of  service 
have  become  a  part  of  our  language.  It  is  of  course  possible 
that  in  his  desire  to  avoid  any  possible  confusion  such  as  might 
arise  through  putting  forward  a  purely  tentative  classification 
he  has  been  overcautious. 

It  is  possible,  further,  that  in  numerous  instances  it  may 
appear  that  too  much  reliance  is  placed  upon  single  analyses, 
particularly  in  the  discussions  relating  to  the  character  of 
decomposed  material.  Regarding  this  it  can  only  be  said  that 
in  those  instances  upon  which  most  reliance  is  placed,  the 
materials  were  not  merely  collected  by  the  author  himself,  but 
that  he  made  his  own  chemical  analyses  and  microscopic  deter- 
minations as  well.  It  is  believed  that  the  fresh  and  residual 
materials  examined  are  in  each  instance  as  truly  representative 
of  the  same  rock  mass,  as  would  be  samples  of  fresh  rock  col- 
lected equal  distances  apart.  In  all  cases  special  effort  was 
made  to  obtain  material  concerning  the  lithological  identity 
of  which  there  could  be  no  doubt,  and  in  the  majority  of  cases 
the  residuary  matter  was  collected  from  positions  immediately 
overlying  the  still  unaltered  rock.  Where  such  a  procedure 


PREFATORY  NOTE  ix 

was  impossible,  especial  care  was  exercised  to  obtain  only  such 
as  was  originally  of  the  same  lithological  nature  as  the  fresh 
rock,  and  which  had  suffered  no  contamination  from  extrane- 
ous sources.  The  fact  that  stratified  rocks  are  likely  to  vary 
so  greatly  within  short  distances,  and  hence  that  a  residual  clay 
cannot  be  relied  upon  to  represent  the  residue  from  rocks  of 
the  same  nature  immediately  underlying,  will  serve  to  explain 
in  part  the  author's  limiting  himself  so  largely  to  a  discussion 
of  massive  eruptive  materials.  That  so  little  use  has  been 
made  of  other  analyses,  made  in  greater  detail  or  by  those  more 
skilled  in  analytical  methods,  is  due  to  a  lack  of  satisfactory 
information  relative  to  the  mutual  association  of  the  fresh  and 
decomposed  materials  and  the  mineralogical  and  physical  nature 
of  the  residual  product. 

As  will  be  readily  perceived  by  those  at  all  acquainted  with 
the  general  literature,  the  publications  of  the  U.  S.  Geological 
Survey,  the  U.  S.  National  Museum,  and  the  Bulletins  of  the 
Geological  Society  of  America  have  been  drawn  upon  to  furnish 
materials  for  illustration.  The  writer  is  under  special  obliga- 
tion to  Dr.  Milton  Whitney  of  the  U.  S.  Department  of  Agri- 
culture for  many  of  the  mechanical  analyses  given,  and  to  Mr. 
L.  H.  Merrill  of  the  Maine  Experiment  Station  for  numerous 
criticisms  and  suggestions. 

To  the  late  Dr.  G.  Brown  Goode  he  is  indebted  for  permis- 
sion to  utilize  photographs  and  specimens  forming  a  part  of 
the  collections  of  the  National  Museum  and  also  for  electro- 
types of  sundry  plates  and  figures  in  its  publications. 

GEORGE   P.    MERRILL. 

U.  S.  NATIONAL  MUSEUM,  January,  1897. 


CONTENTS 

PART  I 

THE    CONSTITUENTS,    PHYSICAL    AND    CHEMICAL 

PROPERTIES,  AND  MODE    OF  OCCURRENCE 

OF  ROCKS 

PAGE 

I.  INTRODUCTORY  :  ROCKS  DEFINED 1 

II.   THE  CHEMICAL  ELEMENTS  CONSTITUTING  ROCKS        ...  4 

III.  THE  MINERALS  CONSTITUTING  ROCKS 9 

IV.  THE  PHYSICAL  AND  CHEMICAL  PROPERTIES  OF  ROCKS      .        .  *  33 

1.  The  Structure  of  Rocks,  macroscopic  and  microscopic          .  33 

2.  The  Specific  Gravity  of  Rocks 43 

3.  The  Chemical  Composition  of  Rocks     .....  44 

4.  The  Color  of  Rocks 45 

V.   THE  MODE  OF  OCCURRENCE  OF  ROCKS 49 

PART  II 

THE  KINDS   OF  ROCKS 

GENERALITIES,  AND  CLASSIFICATION 56 

I.   IGNEOUS  ROCKS  :  ORIGIN  OF,  AND  CLASSIFICATION  ;  RELATION- 
SHIP  EXISTING   BETWEEN   PLUTONIC   AND   EFFUSIVE  ROCKS  59 

1.  The  Granite-Liparite  Group 65 

2.  The  Syenite-Trachyte  Group 73 

3.  The  Foyaite-Phonolite  Group 77 

4.  The  Diorite-Andesite  Group 81 

5.  The  Gabbro-Basalt  Group 85 

6.  The  Theralite-Basanite  Group 93 


xii  CONTENTS 

PAGE 

7.  The  Peridotite-Limburgite  Group  95 

8.  The  Pyroxenite-Augitite  Group 99 

9.  The  Leucite-Nepheline  Rocks 102 

II.   AQUEOUS  ROCKS 105 

1.  Rocks  formed  through  Chemical  Agencies    ....  105 

(1)  Oxides 106 

(2)  Carbonates Ill 

(3)  Silicates 114 

(4)  Sulphates 117 

(5)  Phosphates 119 

(6)  Chlorides .  119 

(7)  Hydrocarbon  Compounds 120 

2.  Rocks  formed  as  Sedimentary  Deposits          ....  129 

(1)  Rocks  composed  mainly  of  Inorganic  Material  .         .  131 

(1)  The  Arenaceous  Group :  Psammites      .         .  131 

(2)  The  Argillaceous  Group  :  Pelites  .         .         .  135 
.                               (3)    The  Calcareous  Group :  Calcareous  Conglom- 
erate and  Breccia 139 

(4)    The  Volcanic  Group  :  Tuffs  .         .         .         .139 

(2)  Rocks  composed  mainly  of  debris  from  Plant  and 

Animal  Life.     Organagenous     ....  141 

(1)  The  Siliceous  Group :  Infusorial  Earth          .  141 

(2)  The  Calcareous  Group :  Limestone,  Marl,  etc.  143 

(3)  The   Carbonaceous    Group  :    Peat,   Lignite, 

and  Coal 148 

(4)  The  Phosphatic  Group 151 

III.  JEOLIAN  ROCKS 153 

Volcanic  Dust ;  Dune  Sands,  etc 153 

IV.  METAMORPHIC  ROCKS 155 

Agencies  and  Results  of  Metamorphism  and  Metasomatosis     .  155 

1.  Stratified  or  Bedded 162 

(1)    The  Crystalline  Limestones  and  Dolomites  .  162 

2.  Foliated  or  Schistose 164 

(1)  The  Gneisses  .         .         .        .        .         .         .164 

(2)  The  Crystalline  Schists 168 


CONTENTS  xiii 


PART  III 
THE   WEATHERING   OF  ROCKS 

PAGE 

I.  THE  PRINCIPLES  INVOLVED  IN  ROCK-WEATHERING  :  Statement 
of  General  Problem  ;  Weathering  defined  ;  Reference  to  Au- 
thorities and  Opinions  held 173 

1.  Action  of  the  Atmosphere 176 

(1)  Nitrogen,  Nitric  Acid,  and  Ammonia  of  the 

Atmosphere 176 

(2)  Carbonic  Acid  of  the  Atmosphere  .         .         .  178 

(3)  Oxygen  of  the  Atmosphere      .         .         .        .180 

(4)  Effects  of  Heat  and  Cold          .         .         .         .180 

(5)  Effects  of  Wind 184 

2.  Chemical  Action  of  Water 186 

(1)  Oxidation 187 

(2)  Deoxidation 187 

(3)  Hydration 187 

(4)  Solution 189 

3.  Mechanical  Action  of  Water  and  of  Ice  ;   Erosion  by 

Water  ;  Daubree's  Experiments  ;  Action  of  Freez- 
ing Water  and  of  Ice  ......  195 

4.  Action  of   Plants  and  Animals ;   Effect  of   Lichens, 

Mosses,  Root  Action,  Organic  Acids,  etc. ;  Solvent 
Power  of  Citric  Acid  ;  Action  of  Bacteria  ;  Action 
of  Ants  and  Termites ;  Action  of  Marine  Inver- 
tebrates ;  Production  of  Carbonates  .  .  .  201 

II.   CONSIDERATION  OF  SPECIAL  CASES 206 

(1-)    Weathering  of  Granite,  District  of  Columbia     .         .         .206 

(2)  Weathering  of  Gneiss,  Albemarle  County,  Virginia   .         .  214 

(3)  Weathering  of  Elseolite  Syenite,  Little  Rock,  Arkansas     .  216 

(4)  Weathering  of  Phonolites,  Marienfels,  Bohemia         .         .  217 

(5)  Weathering  of  Diabase,  Medford,  Massachusetts        .         .  218 

(6)  Weathering  of  Diabase,  Venezuela 222 

(7)  Weathering  of  Basalt,  Kammar  Bull,  Bohemia  .         .         .  223 

(8)  Weathering  of  Basalt,  Haute  Loire,  France        .        .        .  223 


xiv  CONTENTS 

PAGE 

(9)   Weathering  of  Diorite,  Alberaarle  County,  Virginia  .         .  224 

(10)  Weathering  of  Peridotites  and  Pyroxenites        .         .         .  225 

(a)    Serpentine  of  Harford  County,  Maryland  .         .         .  226 
(&)    Soapstones  of  Albernarle  and  Fairfax  Counties,  Vir- 
ginia           226 

(11)  Weathering  of  Clastic  Rocks       .         .         .         .         .         .228 

(a)    Argillites  of  Harford  County,  Maryland    .         .         .229 

(6)    Cherts  of  Missouri  and  Arkansas        ....  230 

(12)  Weathering  of  Limestones,  Arkansas          ....  232 

(13)  Resume:  Importance  of  Hydration  ;  Loss  of  Constituents ; 

Relative  Durability  of  Various  Minerals ;   Discussion  of 

Processes  involved  in  Feldspathic  Decomposition     .         .  234 

HI.   THE  PHYSICAL  MANIFESTATIONS  OF  WEATHERING    .        .        .  241 

(1)  Disintegration  without  Decomposition        ....  241 

(2)  Weathering  influenced  by  Crystalline  Structure         .         .  243 

(3)  Weathering  influenced  by  Structure  of  Rock  Masses .         .  244 

(4)  Weathering  influenced  by  Mineral  Composition          .         .  248 

(5)  Results  due  to  Position 252 

(6)  Induration  on  Exposure 254 

(7)  Changes  in  Color  incidental  to  Weathering        .         .         .  257 

(8)  Relative  Amount  of  Material  removed  in  Solution     .         .  258 

(9)  Incidental  Surface  Contours 259 

(10)  Effacement  of  Original  Characteristics        ....  262 

(11)  Simplification    of    Chemical    Compounds    incidental    to 

Weathering 265 

(12)  Other  Results  incidental  to  Decomposition  and  Erosion     .  266 

IV.   TIME  CONSIDERATIONS 268 

(1)  Rate  of  Weathering  influenced  by  Texture        .         ,         .  268 

(2)  Rate  of  Weathering  influenced  by  Composition          .         .  269 

(3)  Rate  of  Weathering  influenced  by  Humidity     .         ,         .  270 

(4)  Rate  of  Weathering  influenced  by  Position    .'    .         .  .       .270 

(5)  Relative  Rapidity  of  Weathering  among  Eruptive  and  Sedi- 

mentary Rocks          .         .         .         ,         .         .         .         .  271 

(6)  Time  Limit  of  Decay:   Post-Cretaceous  Weathering  of 

Granite;  Weathered  Implements  of  Human  Workman- 
ship ;  Post-Glacial  Weathering  of  Diabase  ;  Post-Jurassic 


CONTENTS  XV 

PAGE 

and  Post-Pliocene  Decay  of  Rocks  of  the  Sierras;  Pre- 
Palaeozoic  Weathering  of  Archaean  Rocks         .         .         .     272 

(7)  Extent    of   Weathering:    In   the  District    of    Columbia, 

Georgia,  Missouri,  Nicaragua,  Brazil,  and  South  Africa    .     276 

(8)  Relative  Rapidity  of  Weathering  in  Warm  and  Cold  Cli- 

mates :     Opinions   hitherto   held  ;    Supposed    Protective 
Action  of  Frost  Effects  of  Forests 278 

(9)  Difference  in  Kind  of  Weathering  in  Cold  and  Warm  Cli- 

mates         283 

(10)    Relative  Amounts  of  Materials  lost  through  Weathering 

in  Hilly  and  Plains  Regions      ......     284 

PART  IV 

TRANSPORTATION  AND  REDEPOSITION   OF  ROCK 
DEBRIS 

1.  ACTION  OF  GRAVITY 286 

2.  ACTION  OF  WATER  AND  ICE 287 

3.  ACTION  OF  WIND 292 

PART  V 

THE  REGOLITH 

I.   CLASSIFICATION  AND  GENERAL  DESCRIPTION     ....     299 

1.  Sedentary  Materials 300 

(1)  Residuary   Deposits :    Residual    Sands    and    Clays ; 

Terra  Rossa  ;  Laterite,  etc 301 

(2)  Cumulose  Deposits  :  Peat ;  Muck  and  Swamp  Soils 

in  part ;  Infusorial  Earths 313 

2.  Transported  Materials 318 

(1)  Colluvial  Deposits  :  Talus,  Cliff  Debris  and  Material 

of  Avalanches         .         .         .         .         .         .         .319 

(2)  Alluvial    Deposits  :    Modern    Alluvium ;    Sea-coast 

Swamps  ;  Loess  ;  Adobe  in  part ;  Champlain  Clays ; 
Beach  Sands  and  Gravel  320 


XVi  CONTENTS 

PAGE 

(3  )   .ZEolian  Deposits  :  Wind-blown  Sand  ;   Sand  Dunes ; 

Volcanic  Dust .344 

(4)  Glacial  Deposits :  Moraine  Material ;  Eskers ;  Drum- 

lins,  etc .  350 

3.  The  Soil .358 

(1)  The  Chemical  Nature  of  Soils 358 

(2)  The  Mineral  Composition  of  Soils     .         .         .         .374 

(3)  The  Physical  Condition  of  Soils        .         .         .         .379 

(4)  The  Weight  of  Soils 382 

(5)  The  Kinds  and  Classification  of  Soils        .         .         .382 

(6)  The  Color  of  Soils 385 

(7)  The  Age  of  Soils 387 

(8)  Soils  as  Affected  by  Plant  and  Animal  Life      .         .     390 


ILLUSTRATIONS 

FULL-PAGE   PLATES 

FACING   PAGE 

PLATE  1 Frontispiece 

Stone  Mountain,  Georgia.      A  Residual  Boss  of  Granite.     From 
a  photograph  by  J.  K.  Killers. 

PLATE  2 .         .  .33 

Porphyritic  and  Flow  Structures. 

PLATE  3        .         . 35 

Slaggy  and  Vesicular  Structures. 

PLATE  4 38 

Brecciated  Structures. 

PLATE  5 41 

Microscopic  Structures  of  Rocks. 

PLATE  6 65 

Fig.  1.  Lithophysse  in  Liparite. 

Fig.  2.  Cross-section  of  Stalagmite. 

Fig.  3.  Concretionary  Aragonite. 

Fig.  4.  Pegmatite. 

PLATE  7  .         . 70 

Fig.  1.  Liparite,  Nevadite  Form. 

Fig.  2.  Liparite,  Rhyolite  Form. 

Fig.  3.  Liparite,  Obsidian  Form. 

Fig.  4.  Liparite,  Pumiceous  Form. 

PLATE  8 82 

Fig.  1.    Orbicular  Diorite. 
Fig.  2.    Granite  Spheroid. 

PLATE  9 .     107 

Fig.  1.    Botryoidal  Hematite. 
Fig.  2.   Septarian  Nodule. 

PLATE  10 113 

View  in  Limestone  Cavern. 

xvii 


XViii  ILLUSTRATIONS 

FACING  PAGE 
PLATE  11 .     130 

Fig.  1.   Shell  Limestone. 

Fig.  2.    Coquina. 

Fig.  3.    Crinoidal  Limestone. 

PLATE  12 143 

Fig.  1.    Pisolitic  Limestone. 
Fig.  2.    Oolitic  Limestone. 

PLATE  13 164 

Banded  and  Foliated  Gneisses. 

PLATE  14 172 

Weathered  Granite,  District  of  Columbia.     From  a  photograph 
by  George  P.  Merrill. 

PLATE  15 193 

Corroded  Limestones. 

PLATE  16 199 

Fig.  1.  Diorite  Boulder  split  along  Joint  Planes  by  Frost. 

Fig.  2.  Corroded  Surface  of  Pyroxenic  Limestone. 

Fig.  3.  Corroded  Limestone. 

PLATE  17 219 

Weathered  Diabase  Dike,  Medf'ord,  Mass.    From  a  photograph  by 
G.  H.  Barton. 

PLATE  18 241 

Fig.  1.    Exfoliated  Granite  in  the  Sierras.      From  a  photograph 

by  H.  W.  Turner. 
Fig.  2.    Talus  Slopes  on  Pike's  Peak.      From  a  photograph  by  W. 

H.  Jackson. 
Fig.  3.    Disintegrated    Granite,    Ute   Pass,    Colorado.      From   a 

photograph  by  W.  H.  Jackson. 

PLATE  19 .248 

Fig.  1.  Weathered  Schists,  Coast  of  Cape  Elizabeth,  Maine. 

Fig.  2.  Sandstone  bored  by  Bees. 

Fig.  3.  Slab  of  Glaciated  Limestone. 

PLATE  20 258 

Fig.  1.   Weathered  Boulder  of  Oriskany  Sandstone. 
Fig.  2.    Concentric  Weathering  in  Diabase. 
Fig.  3.   Zonal  Structure  in  Weathered  Argillite. 
Fig.  4.    Weathered   Sandstone   showing   Induration   along  Joint 
Planes. 


ILLUSTRATIONS  xix 

FACING    PAGK 

PLATE  21      .  .  ...     267 

Fig.  1.    Sink-hole  near  Knoxville,  Tennessee.     From  a  photograph 

by  George  P.  Merrill. 
Fig.  2.    Beds  of  Marble  corroded  by  Meteoric  Waters,  Pickens 

County,  Georgia. 

PLATE  22 285 

Fig.  1.    Forest  destroyed  by  Wind-blown  Sand.     From  a  photo- 
graph by  I.  C.  Russell. 

Fig.  2.    Calcareous  Conglomerate  carved  and  polished  by  Wind- 
blown Sand. 

Fig.  3.    Rock  being  undermined  by  Wind-blown  Sand.     After  G. 
K.  Gilbert. 

PLATE  23 319 

Rock  Disintegration  and   Formation  of   Talus,  Mount  Sneffels, 
Colorado.     From  a  photograph  by  Whitman  Cross. 

PLATE  24 345 

Fig.  1.    Section  of  Beds  of  Leda  Clay,  Lewiston,  Maine.     From  a 

photograph  by  L.  H.  Merrill. 
Fig.  2.    Beds  of  Volcanic  Dust,  Reese  Creek,   Gallatin  County, 

Montana.     From  a  photograph  by  George  P.  Merrill. 

PLATE  25 357 

Fig.  1.    Section  of  Glacial  Till.     From  a  photograph  by  G.  F. 

Wright. 
Fig.  2.    Glaciated   Landscape.      From  a  photograph   by   L.    H. 

Merrill. 

Plates  2,  3,  4,  5,  6,  7,  8,  9,  11,  12,  13,  15,  19,  and  20,  and  Fig.  3  Plate  16, 
and  Fig.  2  Plate  22,  from  specimens  in  the  Geological  Department  of 
the  United  States  National  Museum. 


FIGURES  IN   TEXT 

'IG.  PAGE 

1.  Augite  partially  altered  into  Hornblende 40 

2.  Mounted  Thin  Section  of  Rock 43 

3.  Microscopic  Structure  of   Muscovite-Biotite  Granite,    Hallowell, 

Maine 67 

4.  Microscopic  Structure  of  Diabase,  Weehawken,  New  Jersey  .         .  88 

5.  Microscopic  Structure  of  Peridotite  (Porphyritic  Lherzolite)         .  96 

6.  Microscopic  Structure  of  Pyroxenite 100 


XX  ILLUSTRATIONS 

FIG.  PAGE 

7.  Microscopic  Structure  of  Oolitic  Limestone 112 

8.  Pyroxene  partially  altered  into  Serpentine 115 

9.  Microstructure  of  Sandstone 131 

10.  Section  through  Lake  Basin,  showing  Bed  of  Infusorial  Earth      .  142 

11.  Microstructure  of  Oolitic  Limestone        ......  144 

12.  Microstructure  of  Fossiliferous  Limestone      .....  145 

13.  Microstructure  of  Quartzite     ........  158 

14.  Microstructure  of  Crystalline  Limestone         .....  163 

15.  Microstructure  of  Gneiss 165 

16.  Microstructure  of  Quartzite 169 

17.  Influence  of  Joints  in  the  Production  of  Boulders  ....  244 

18.  Exfoliation  of  Granite,  Stone  Mountain,  Georgia  ....  245 

19.  Concentric  Exfoliation  of  Granite,  Canada 246 

20.  Microstructure  of  Sandstone,  with  Large  Absorptive  Power  .         .  269 

21.  Microstructure  of  Diabase,  with  relatively  Little  Absorptive  Power  269 

22.  Flint  Implement  showing  Weathered  Surface        ....  274 

23.  Sketch  showing  Pre-Palseozoic  Decay  of  Rocks       ....  276 

24.  Diagram  showing  Direction  and  Rate  of  Motion  of  Soil        .        .  287 

25.  Diagram  showing  Flood  Plain  of  River 289 

26.  Angular  Outlines  of  Particles  in  Residual  Soil  from  Gneiss  .         .  301 

27.  Section  across  Central  Kentucky,  showing  Inherited  Characteris- 

tics of  Soils 303 

28.  Angular  Quartz  Particles  from  Decomposed  Gneiss        .         .         .  304 

29.  Outlines  of  Kaolinite  Crystals  and  Kaolin  Particles        .         .         .  309 

30.  Section  across  Small  Lake 314 

31.  Talus  Slopes 319 

32.  Alluvial  Plains 323 

33.  Outlines  of  Particles  in  Chinese  Loess ,329 

34.  Particles  washed  from  Leda  Clays 335 

35.  Cross-section  of  Marine  Marsh 338 

36.  Quartz  Granules  in  Beach  Sand      .......  343 

37.  Outlines  of  Particles  of  Glass  in  Volcanic  Dust  .        .        .  349 

38.  Section  through  Carboniferous  Soil 386 

39.  Section  showing  Varying  Character  of  Residual  Soil     .         .         .  387 

40.  Section  through  Ant  Nest 390 

41  and  42.     Sections  showing  the  Effect  of  Tree  Roots  in  Soil     .        .395 

Fig.  1,  after  G.  W.  Hawes ;  5  and  6,  after  G.  H.  Williams ;  18  and  22, 

after  Robert  Bell ;   10,  23,  24,  26,  29,  30,  31,  34,  37,  38,  39,  40,  and  41,  after 
Shaler,  Twelfth  Annual  Report  United  States  Geological  Survey,  1890-1891. 


ROCKS,  ROCK-WEATHERING, 
AND  SOILS 

PART   I 

THE  CONSTITUENTS,  PHYSICAL  AND  CHEMICAL 
PROPERTIES,  AND  MODE  OP  OCCURRENCE  OP 
ROCKS 

I.    INTRODUCTORY 

A  ROCK  is  a  mineral  aggregate ;  more  than  this,  it  is  an 
essential  portion  of  the  earth's  crust,  a  geological  body  occu- 
pying a  more  or  less  well-defined  position  in  the  structure  of 
the  earth,  either  in  the  form  of  stratified  beds,  eruptive  masses, 
sheets  or  dikes,  or  in  that  of  veins  and  other  chemical  deposits 
of  comparatively  little  importance  as  regards  size  and  extent. 
In  giving  this  definition,  origin,  chemical  composition,  and  state 
of  aggregation  of  the  individual  particles  are  for  the  time 
ignored.  From  a  strictly  geological  standpoint,  the  beds  of 
loose  sand,  and  even  the  water  of  the  ocean  itself,  may  be 
considered  as  rocks,  and  either,  under  favorable  circumstances, 
may  undergo  a  process  of  induration  such  as  shall  be  produc- 
tive of  the  condition  of  solidity  commonly  ascribed  to  rocks 
by  the  popular  mind. 

In  ever-varying  conditions  as  regards  compactness,  color, 
texture,  and  structure,  rocks  form  the  entire  mass  of  the  globe 
so  far  as  it  is  as  yet  made  known  to  us,  with  the  exception  of  a 
scarcely  appreciable  proportion  of  organic  matter.  It  is  rock 
which  forms  the  substance  of  mountain  ranges  and  the  vast 
stretches  of  valley  and  plain.  It  is  from  the  rocks  that  we 
gain  our  food,  our  fuel,  and  the  supplies  of  metal  which  are 
seemingly  so  essential  to  our  well-being;  we  cannot  ignore 

B  1 


2  INTRODUCTORY 

them,  even  if  we  would.  We  borrow  from  the  rocks  that 
which  is  essential  to  our  life  to-day,  but  when  that  brief  day 
is  ended  return  it  once  more,  with  neither  loss  nor  gain,  to  its 
original  source. 

Those  portions  of  the  earth's  crust  which  are  available  for 
study  comprise  at  best  but  a  few  thousand  vertical  feet,  though 
from  the  fact  that  the  stratified  rocks  have  been  so  extensively 
thrown  out  of  their  original,  horizontal  position,  and  again 
eroded,  we  are  enabled  to  measure  their  thickness,  and  may 
hence  claim  to  know  with  a  reasonable  degree  of  accuracy  the 
character  of  the  material  forming  this  crust  down  to  a  depth 
of  perhaps  twenty  miles.1  Throughout  all  this  vast  thickness, 
comprising  millions  upon  millions  of  cubic  feet,  in  weight  far 
beyond  all  comprehension,  we  find  a  constant  recurrence  of 
materials  alike  in  composition  and  similarity  in  origin  to  those 
upon  the  immediate  surface.  There  is  at  times,  as  noted  later, 
a  difference  in  structure  due  to  metamorphism,  between  the 
older,  deeper  lying  portions  and  those  more  recent,  but  the 
ultimate  composition  is  essentially  the  same,  and  all  the  know- 
ledge thus  far  gained  points  to  a  wonderful  unity  in  nature's 
methods,  and  shows  with  seeming  conclusiveness  that  the  geo- 
logical agencies  of  the  past,  the  methods  by  which  rocks  were 
made  and  again  destroyed,  differed  in  no  essential  particular 
from  those  in  progress  to-day.  What  these  processes  were, 
how  they  operated,  and  with  what  results,  it  shall  be  our  aim 
to  here  set  forth. 

Among  the  many  interesting,,  and  at  first  thought  seemingly 
unaccountable,  things  we  shall  encounter  in  the  progress  of 
our  work,  not  the  least  is  the  fact  that  so  large  a  proportion 
of  natural  objects  are  more  or  less  out  of  harmony  with  their 
surroundings.  Throughout  life  every  organic  being  is  in  a 
constant  struggle  with  the  elements  to  preserve  that  life,  fulfil 
all  its  functions,  and  gratify  its  natural  desires.  No  sooner 
does  life  depart  than  decomposition  and  disintegration  ensue. 
As  with  organic  beings,  so  with  inorganic  substances.  Every 
mass  of  rock  pushed  up  by  the  faulting  and  folding  of  the 
earth's  crust,  exposed  by  denudation,  or  erupted  as  molten 
matter  from  the  earth's  interior,  finds  almost  at  once  that  its 
various  elements,  in  their  existing  combinations,  are  not  in  har- 

1  The  total  mean  depth  of  the  fossiliferous  formations  of  Europe  as  stated  by 
Geikie  (Text-book  of  Geology,  p.  675)  has  been  set  down  as  75,000  feet. 


INTRODUCTORY  3 

mony  with  their  environment.  The  summer's  heat  and  winter's 
cold,  the  chemical  action  of  atmospheres  and  acidulated  rains, 
combine  their  forces  ;  a  breaking  up  ensues,  to  be  succeeded 
by  new  combinations  and  perhaps  reconsolidations  more  in 
keeping  with  the  then  existing  circumstances.  An  intermedi- 
ate product  in  all  this  endless  cycle  of  change,  of  disintegration 
and  recombination,  is  a  comparatively  thin,  superficial  mantle 
of  loose  debris,  which,  mixed  with  more  or  less  organic  matter, 
nearly  everywhere  covers  the  land,  and  by  its  combined  chemi- 
cal and  mechanical  properties  furnishes  food  and  foothold  for 
myriads  of  plants,  and  hence,  indirectly,  sustenance  for  man  and 
beast  as  well.  In  brief,  what  is  commonly  known  as  soil  is  but 
disintegrated  and  more  or  less  decomposed  rock  material,  inter- 
mingled, perhaps,  with  organic  matter  from  plant  decay.  Such 
being  the  case,  a  study  of  the  processes  of  rock  weathering  and 
the  transportation,  deposition,  and  physical  properties  of  the 
resultant  debris,  is  but  a  study  of  the  origin  of  soils  on  the 
broadest  and  most  comprehensive  basis,  and  soils  themselves 
may  justly  be  regarded  as  secondary  rocks  in  a  state  of  in- 
complete consolidation.  Their  study  belongs,  therefore,  as 
legitimate^  to  the  realm  of  geology  as  does  that  of  any  sub- 
ject relating  to  rock  formation  or  other  phases  of  the  earth's 
history. 

Accepting  the  above,  we  will  begin  our  studies  by  a  consid- 
eration of  (1)  the  elements  which  in  their  single  or  combined 
state  make  up  the  minerals  ;  (2)  the  minerals  which  make  up 
the  rocks ;  (3)  the  rocks  themselves,  with  particular  reference 
to  their  mineralogical  and  chemical  natures ;  (4)  the  breaking 
down  or  degeneration  of  rocks  through  processes  in  part  chemi- 
cal and  in  part  mechanical ;  and  (5)  the  result  of  this  clasmatic 
process  as  manifested  in  the  production  of  clay,  sand,  gravel, 
and  incidental  soil.  There  are  other  points  which  will  be 
touched  upon  more  briefly,  in  order  to  make  our  work  system- 
atic, as  the  action  of  wind  and  water  in  assorting  and  redeposit- 
ing  rock  debris  and  tending  to  reduce  the  land  surface  to  one 
general  level. 


II.    THE   CHEMICAL   ELEMENTS   CONSTITUTING 

ROCKS 

Although  there  are  69  elements  now  known,  but  16  occur  in 
any  abundance  or  form  more  than  an  extremely  small  proportion 
of  the  material  of  the  earth's  crust.  Indeed,  of  this  number 
probably  fully  one-half,  taken  collectively,  will  not  constitute 
more  than  4  or  5%  of  the  earth's  crust  so  far  as  known.  These 
16,  arranged  according  to  their  chemical  properties  and  order 
of  their  abundance,  are  as  follows  :  oxygen,  silicon,  carbon, 
sulphur,  hydrogen,  chlorine,  phosphorus,  fluorine,  aluminum, 
calcium,  magnesium,  potassium,  sodium,  iron,  manganese,  and 
barium.  The  eight  more  important,  with  their  approximate 
percentage  amounts  as  given  by  Roscoe  and  Schorlemmer,1  are 

as  below  :  — 

Oxygen 44.0  to  48.7% 

Silicon 22.8  to  36.2 

Aluminum 9.9  to    6.1 

Iron 9.9  to    2.4 

Calcium 6.6  to    0.9 

Magnesium 2.7  to    0.1 

Sodium 2.4  to    2.5 

Potassium 1.7  to    3.1 

It  must  not  for  a  moment  be  imagined,  however,  that  these 
elements  exist  for  the  most  part  in  a  free  or  uncombined  state : 
on  the  contrary,  in  the  majority  of  cases  so  great  is  their  affinity 
for  one  another  that  it  is  only  momentarily,  or  under  abnormal 
conditions,  that  they  are  met  with  at  all  in  this  elementary 
form.  Those  elements  which  are  most  common  in  the  free 
state,  though  even  these  occur  more  commonly  combined  with 
others,  are,  (1)  the  gas  oxygen,  and  (2)  the  solids,  carbon,  sul- 
phur, and,  more  rarely,  iron.  Still  more  rarely,  and  under  such 
abnormal  conditions,  as  exist  during  volcanic  eruptions,  are  found 
the  free  gases,  hydrogen,  chlorine,  and  fluorine.  The  gas  nitro- 
gen, although  so  abundant  a  constituent  of  the  atmosphere, 

1  Treatise  on  Chemistry,  Vol.  I,  p.  55,  1878. 
4 


OXYGEN  5 

is,  as  a  primary  constituent  of  the  earth's  crust,  almost  wholly 
unknown,  and  needs  no  consideration  at  this  stage  of  our 
work. 

Oxygen,  as  is  well  known,  is  the  active,  even  the  aggressive, 
principle  of  the  atmosphere,  of  which  it  constitutes  about  one- 
fifth  by  bulk.  Combined  with  other  elements,  it  is,  however, 
of  vastly  greater  geological  importance,  being  estimated,  as 
noted  above,  to  constitute  from  44  to  48.7%  of  the  entire  mass 
of  the  earth's  crust ;  that  is  to  say,  could  the  earth's  crust  be 
once  more  resolved  into  its  original  elements,  the  oxygen  thus 
liberated  would  be  found  very  nearly  equal  to  all  the  other 
elements  taken  together.  The  simpler  forms  of  oxygen  com- 
pounds are  known  as  oxides,  and  of  these  the  oxide  of  hydrogen, 
water  (H2O),  is  by  far  the  most  common,  and,  anomalous  as  it 
may  at  first  seem,  is  a  true  mineral  and  to  be  classed  as  an 
anhydrous  oxide  at  that.  Aside  from  being  so  essential  to 
human  life,  oxygen,  as  will  be  noted  later,  is  a  very  potent 
factor  in  the  manifold  changes  which  are  constantly  taking 
place  in  the  more  superficial  portions  of  the  earth's  crust. 

Silicon.  —  Next  to  oxygen  silicon  is  the  most  abundant  of 
the  earth's  constituents,  though  it  exists  only  in  combination, 
either  as  an  oxide  (SiO2),  or  with  other  elements  to  form 
silicates.  In  these  two  forms  it  is  the  predominating  con- 
stituent in  all  but  the  calcareous  rocks.  As  silica  (SiO2),  or 
quartz,  it  forms  one  of  the  most  indestructible  of  natural  com- 
pounds, and  hence  is  to  be  found  as  the  prevailing  constituent 
in  nearly  all  sands  and  soils. 

Aluminum  is  next  to  oxygen  and  silicon  probably  the  most 
important  element  when  regarded  from  our  present  standpoint. 
It  occurs  mainly  in  combination  with  silicon  and  oxygen,  form- 
ing an  important  series  of  minerals  known  as  aluminous  sili- 
cates. As  a  sesquioxide  it  is  well  known  in  the  minerals 
corundum  and  beauxite. 

Iron,  although  less  abundant  than  either  oxygen  or  silica, 
occupies  a  very  important  place  as  a  rock  constituent,  owing  to 
the  variety  of  compounds  of  which  it  forms  a  part,  as  well  as 
to  the  decided  colors  which  are  characteristic  of  its  oxides  and 
of  the  iron-bearing  silicates.  The  most  conspicuous  forms  of  iron 
on  the  immediate  surface  of  the  earth  are  the  oxides,  but  which 
at  greater  depths,  or  where  the  atmosphere  has  as  yet  exercised 
less  influence,  give  way  to  carbonates,  sulphides,  and  silicates. 


6  CHEMICAL  ELEMENTS   CONSTITUTING  THE    ROCKS 

Iron,  although  so  common  in  combination  with  other  ele- 
ments, occurs  but  rarely  free,  owing  to  its  affinity  for  oxygen. 
It  is  possible  that  far  below  the  surface,  beyond  the  reach 
of  meteoric  waters  and  atmospheric  air,  it  is  to  be  found  in 
a  metallic  state  much  more  abundantly,  but  of-  this  we  have 
no  other  proof  than  that  the  specific  gravity  of  the  globe,  in 
its  entirety,  is  much  greater  than  that  of  the  most  dense  minerals 
which  constitute  its  outer  portion.  The  inference  seems  un- 
avoidable that  at  great  depths  some  of  these  elements  exist 
uncombined,  and  in  a  state  of  greater  molecular  density  than 
at  the  surface. 

Calcium  is  a  very  important  element  of  the  earth's  crust, 
although,  as  we  have  seen,  it  has  been  estimated  to  compose 
only  about  one-sixteenth  of  its  mass.  Its  most  conspicuous 
form  of  occurrence  is  in  combination  with  carbon  dioxide, 
forming  the  mineral  calcite  (CaCO3),  or  the  rock  limestone. 
In  this  form  it  is  slightly  soluble  in  water  containing  carbonic 
acid,  and  hence  has  become  an  almost  universal  ingredient  of 
all  natural  waters,  whence  it  furnishes  the  lime  necessary  for 
the  formation  of  shells  and  skeletons  of  the  various  tribes  of 
mollusca  and  corals.  In  combination  with  sulphuric  acid, 
calcium  forms  the  rock  gypsum.  It  is  also  an  important  con- 
stituent of  many  silicates. 

Magnesium  is  found  in  combination  with  carbonic  acid  as 
carbonate,  forming  thus  an  essential  part  of  the  rock  dolomite. 
The  bitter  taste  of  sea-water  and  some  mineral  waters  is  due  to 
the  presence  of  salts  of  magnesia.  In  combination  with  silica 
as  a  silicate  it  forms  an  essential  part  of  such  rocks  as  serpen- 
tine, soapstone,  and  talc. 

Potassium  combined  with  silica  is  also  an  important  element 
in  many  mineral  silicates,  as  orthoclase,  leucite,  and  nepheline. 
In  smaller  amounts  it  is  found  in  silicates  of  the  mica,  amphi- 
bole,  and  pyroxene  groups.  The  following  table  will  serve  to 
show  the  varying  amounts  of  potash  (K2O)  in  rocks  of  various 
kinds  :  — 

Granite 2.6  to  6.50% 

Diorite 0.1  to  2.42% 

Basalt 0.058 to  0.50% 

Gabbro 0.00  to  0.93% 

Limestone ,  0.19  to  1.22% 

Sandstone 0.00  to  3.30% 

Slate  (fissile  argillite) 0.00  to  3.83% 


SODIUM  7 

As  a  chloride,  potassium  is  invariably  present  in  sea-water, 
and  as  a  nitrate  it  forms  the  rare,  but  valuable  mineral  nitre,  or 
saltpetre. 

Sodium.  —  The  most  common  and  wide-spread  form  of  the 
element  sodium  is  the  compound  with  chlorine  known  as 
sodium  chloride  (NaCl)  or  common  salt.  In  this  form  it  is 
the  most  abundant  of  the  salts  occurring  in  sea-water,  and 
constitutes  also  rock  masses  of  no  inconsiderable  dimensions 
interstratified  Avith  other  rocks  of  the  earth's  crust.  Combined 
with  silica,  lime,  and  alumina,  sodium  is  an  important  constitu- 
ent of  the  soda-lime  feldspars,  and  of  numerous  other  silicate 
minerals.  In  the  form  of  carbonate  and  sulphate  it  occurs  as 
an  incrustation  on  the  surface,  or  disseminated  throughout  the 
soils  in  poorly  drained  portions  of  arid  countries,  giving  rise  to 
the  so-called  "alkali  soils,"  for  which  such  regions  are  frequently 
noted.  As  a  nitrate,  sodium  occurs  in  the  desert  regions  of 
Chili,  forming  the  soda  nitre  so  valuable  for  fertilizing  purposes. 

Manganese  is,  next  to  iron,  the  most  abundant  of  the  heavy 
metals,  occurring  as  oxide,  carbonate,  or  in  combination  with 
two  or  more  other  elements  as  a  silicate. 

Barium  is  found  mainly  combined  with  sulphuric  acid,  to 
form  the  mineral  barite  or  heavy  spar.  It  sometimes  occurs 
as  a  carbonate,  and  more  rarely  as  a  silicate. 

Phosphorus,  although  existing  in  comparatively  insignificant 
proportions,  is  nevertheless  an  important  element,  though  in 
nature  it  occurs  only  in  combination  with  various  bases,  prin- 
cipally lime,  to  form  phosphates.  In  this  form  it  is  found  in 
the  bones  of  animals,  the  seeds  of  plants,  and  constitutes  the 
essential  portions  of  the  minerals  apatite  and  phosphorite. 
Though  small  in  proportion,  phosphorus  is  a  very  important 
constituent  of  any  fertile  soils.  Its  chief  source,  in  the  older, 
crystalline  rocks,  is  the  mineral  apatite,  as  noted  later.  As 
found  in  the  secondary  rocks,  as  limestones  and  marls,  it  is 
evidently  derived  from  animal  remains.  (Seep.  151.)  Analy- 
ses have  shown  that  the  amount  of  phosphorus,  in  the  form  of 
phosphoric  anhydride  (P2O5),  in  rocks  rarely  exceeds  1%,  and 
usually  falls  much  lower,  being  most  abundant  in  the  basic 
eruptive  rocks,  as  diorites  and  gabbros,  and  most  lacking  in 
the  siliceous  fragmentals,  as  sandstones  and  slates.  The  fol- 
lowing table  will  serve  to  show  the  small  percentages  of  this 
constituent  in  rocks  of  various  kinds:  — 


8  CHEMICAL  ELEMENTS   CONSTITUTING  THE   ROCKS 

Granite 0.07  to    0.25% 

Diorite 0.18  to    1.06% 

Basalt 0.03  to    1.18% 

Limestone 0.06  to  10.00% 

Shale 0.02  to    0.25% 

Sandstone 0.00  to    0.1  % 

Of  the  solid  elements  occurring  free,  or  uncombined,  carbon 
is  by  far  the  more  abundant,  being  found  in  the  forms  known 
as  diamond  and  graphite,  or  when  quite  impure  as  coal.  In 
combination  as  a  dioxide  (CO2),  it  forms  the  well-known  car- 
bonic acid  gas,  which,  like  oxygen,  is  a  powerful  agent  in 
bringing  about  important  changes  in  the  rocks  with  which  it 
comes  in  contact.  Free  sulphur  occurs  more  rarely,  being  as  a 
rule  a  product  of  volcanic  activity,  or  due  to  the  reduction  of 
the  sulphides  and  sulphates  of  the  metal  with  which  it  more 
commonly  exists  in  combination. 


III.     THE   MINERALS   CONSTITUTING   ROCKS 

A  rock,  as  previously  stated,  is  a  mineral  aggregate.  As  a 
rule,  the  number  of  mineral  species  constituting  any  essential 
portion  of  a  rock  is  very  small,  seldom  exceeding  three  or  four. 
In  common  crystalline  limestones,  the  only  essential  constitu- 
ent is  the  mineral  calcite ;  granite,  on  the  other  hand,  is, 
as  a  rule,  composed  of  minerals  of  three  or  four  independent 
species.  As  has  been  elsewhere  stated,  the  mineral  composition 
of  rocks  in  general  is  greatly  simplified  by  the  wide  range  of 
conditions,  under  which  the  commonest  minerals  can  be  formed, 
thus  allowing  their  presence  in  rocks  of  all  classes  and  of  what- 
ever origin.  Thus  quartz,  feldspar,  mica,  the  minerals  of  the 
hornblende  or  pyroxene  group,  can  be  formed  in  a  mass  cooling 
from  a  state  of  fusion  ;  they  may  be  crystallized  from  solution, 
or  be  formed  from  volatilized  products.  They  are  therefore 
the  commonest  of  minerals  and  rarely  excluded  from  rocks  of 
any  class,  since  there  is  no  process  of  rock  formation  which 
determines  their  absence.  Moreover,  most  of  the  common 
minerals,  like  the  feldspars,  micas,  hornblendes,  pyroxenes,  and 
the  alkaline  carbonates,  possess  the  capacity  of  adapting  them- 
selves to  a  very  considerable  range  of  compositions.  In  the 
feldspars,  for  example,  the  alkalies,  lime,  soda,  or  potash  may 
replace  each  other  almost  indefinitely,  and  it  is  now  commonly 
assumed  that  true  species  do  not  exist,  all  being  but  isomorphous 
admixtures  passing  into  one  another  by  all  gradations,  and  the 
names  albite,  oligloclase,  anorthite,  etc.,  are  to  be  used  only  as 
indicating  convenient  stopping  and  starting  points  in  the  series. 
Hornblende  or  pyroxene,  further,  may  be  pure  silicates  of  lime 
and  magnesia,  or  iron  and  manganese  may  partially  replace  these 
substances.  Lime  carbonate  may  be  pure,  or  magnesia  may 
replace  the  lime  in  any  proportion.  These  illustrations  are 
sufficient  to  indicate  the  reason  of  the  great  simplicity  of  rock 
masses  as  regards  their  chief  constituents,  and  that  whatever 
may  be  the  composition  of  a  mass  within  nature's  limits,  and 

9 


10  THE   MINERALS   CONSTITUTING  ROCKS 

whatever  may  be  the  conditions  of  its  origin,  the  probabilities 
are  that  it  will  be  formed  essentially  of  one  or  more'  of  a  half 
a  dozen  minerals  in  some  of  their  varieties. 

But  however  great  the  adaptability  of  these  few  minerals  may 
be,  they  are,  nevertheless,  subject  to  very  definite  laws  of  chemi- 
cal equivalence.  There  are  elements  which  they  cannot  take 
into  their  composition,  and  there  are  circumstances  which  retard 
their  formation  while  other  minerals  may  be  crystallizing.  In 
a  mass  of  more  or  less  accidental  composition  it  may,  there- 
fore, be  expected  that  other  minerals  will  form  in  consider- 
able numbers,  but  minute  quantities.  It  is  customary  to  speak 
of  those  minerals  which  form  the  chief  ingredients  of  any 
rock,  and  which  may  be  regarded  as  characteristic  of  any 
particular  variety,  as  the  essential  constituents,  while  those 
which  occur  in  but  small  quantities,  and  whose  presence  or 
absence  does  not  fundamentally  affect  its  character,  are  called 
accessory  constituents.  The  accessory  mineral  which  predomi- 
nates, and  which  is,  as  a  rule,  present  in  such  quantities  as  to 
be  recognizable  by  the  unaided  eye,  is  the  characterizing  acces- 
sory. Thus  a  biotite  granite  is  a  stone  composed  of  the  essential 
minerals  quartz  and  potash  feldspar,  but  in  which  the  accessory 
mineral  biotite  occurs  in  such  quantities  as  to  give  a  definite 
character  to  the  rock.  The  minerals  of  rocks  may  also  be  con- 
veniently divided  into  two  groups,  according  as  they  are  prod- 
ucts of  the  first  consolidation  of  the  mass  or  of  subsequent 
changes.  This  is  the  system  here  adopted.  We  thus  have :  — 

(1)  The  original  or  primary  constituents,  those  which  formed 
upon  its  first  consolidation.     All  the  essential  constituents  are 
original,  but,  on  the  other  hand,  all  the  original  constituents 
are  not  essential.     Thus,  in  granite,  quartz  and  orthoclase  are 
both  original  and  essential,  while  beryl  and  zircon  or  apatite, 
though  original,  are  not  essential. 

(2)  The  secondary  constituents  are  .those  which  result  from 
changes  in  a  rock  subsequent  to  its  first  consolidation,  changes 
which  are  due  in  great  part  to  the  chemical  action  of  percolat- 
ing water.     Such  are  the  calcite,  chalcedony,  quartz,  and  zeo- 
lite deposits  which  form  in  the  druses  and  amygdaloidal  cavities 
of  traps  and  other  rocks. 

Below  is  given  a  list  of  the  more  important  rock-forming 
minerals,  arranged  as  above  indicated.  Although  these  are 
sufficiently  described  as  regards  their  chemical  and  crystallo- 


ROCK-FORMING  MINERALS 


11 


graphic  properties  in  any  of  the  mineralogies,  it  has  seemed 
advisable  to  devote  some  space  here  to  a  reconsideration  of 
those  most  prominent  as  rock  constituents,  in  order  that  the 
individual  characteristics  of  the  rocks  of  which  they  form  a 
part  may  be  better  understood.  In  passing  them  in  review 
we  will  also  note  briefly  the  characteristic  alteration  and  de- 
composition products  to  which  they  give  rise,  though  the  cause 
of  such  changes  must  be  left  for  another  chapter. 


A.  ORIGINAL  MINERALS. 


1.  Quartz. 

2.  The  Feldspars. 
2  a.    Orthoclase. 
2  b.    Microcline. 
2  c.    Albite. 

2  d.    Oligoclase. 
2  e.    Andesite. 
2/.     Labradorite. 
2  g.    Bytownite. 

2  h.    Anorthite. 

3.  The  Amphiboles. 

3  a.    Hornblende. 
3  b.    Tremolite. 

3  c.    Actinolite. 
3  d.    Arvedsonite. 

3  e.    Glaucophane. 
3/.     Smaragdite. 

4.  The  Monoclinic  Pyroxenes. 

4  a.    Malacolite. 
4  b.    Diallage. 
4c.    Augite. 

4  d.    Acmite. 

4  e.    JSgerite. 

.">.    The  Rhombic  Pyroxenes. 

5  a.    Enstatite  (Bronzite). 
5  b.    Hypersthene. 

6.   The  Micas. 


6  a.    Muscovite. 
66.    Biotite. 
6  c.    Phlogopite. 

7.  Calcite  (and  Aragonite). 

8.  Dolomite. 

9.  Gypsum. 

10.  Olivine. 

11.  Garnet. 

12.  Epidote. 

13.  Zoisite. 

14.  Andalusite. 

15.  Staurolite. 

16.  Scapolite. 

17.  Elseolite  and  Nephelirie. 

18.  Leucite. 

19.  Sodalite. 

20.  Hauyn  (nosean). 

21.  Apatite. 

22.  Menaccanite. 

23.  Magnetite. 

24.  Hematite. 

25.  Chromite. 

26.  Halite  (common  salt). 

27.  Muorite. 

28.  Graphite. 

29.  Carbon. 

30.  Pyrite. 


B.    SECONDARY   MINERALS. 


1.   Quartz. 

1  a.    Chalcedony. 


16. 
Ic. 


Opal. 
Tridymite. 


12 


THE   MINERALS   CONSTITUTING  KOCKS 


2.  Albite. 

3.  The  Amphlboles. 
3  a.    Hornblende. 
3  6.    Tremolite. 

3  c.    Actinolite. 
3d.    Uralite. 

4.  Muscovite  (Sericite). 

5.  The  Chlorites. 
5  a.    Jeff erisite. 
5  6.    Ripidolite. 
5  c.    Penninite. 
5  d.    Prochlorite. 

6.  Calcite  (and  aragonite). 

7.  Wollastonite. 

8.  Scapolite. 

9.  Garnet. 

10.  Epidote. 

11.  Zoisite. 

12.  Serpentine. 

13.  Talc. 

14.  Glaueonite. 

15.  Kaolin. 


16.  The  Zeolites. 
16  a.    Pectolite. 
16  6.    Laumontite 
16  c.    I^renite. 
16  d.    Ttiortisonite 
16  e.    Natrolite. 
16/.    Analcite. 
16  g.    Datolite. 
16  h.    Chabazite. 
16  i.     Stilbite. 

16  k.    Heulandite. 
16 1     Phillipsite. 
16m.  Ptilolite. 
16  n.    Mordenite. 
16  o.    Harmotome 

17.  Hematite. 

18.  Limonite. 

19.  Gothite. 

20.  Turgite. 

21.  Pyrite. 

22.  Marcasite. 


Quartz.  — Composition :  Pure  silica,  SiO2;  specific  gravity  2.6; 
hardness,  7.1 

This  is  one  of  the  commonest  and  most  widely  distributed 
minerals  of  the  earth's  crust,  and  forms  an  essential  constituent 
in  a  variety  of  eruptive  and  sedimentary  rocks,  such  as  granite, 

1  For  convenience  in  determining  minerals,  the  "scale  of  hardness"  given 
below  has  been  adopted  by  mineralogists.  By  means  of  it  one  is  enabled  to 
designate  the  comparative  hardness  of  minerals  with  ease  and  definiteness. 
Thus,  in  saying  that  serpentine  has  a  hardness  equal  to  4,  is  meant  that  it  is  of 
the  same  hardness  as  the  mineral  fluorite,  and  can  therefore  be  cut  with  a  knife, 
but  less  readily  than  calcite  or  marble. 

1.  Talc :  Easily  scratched  with  the  thumbnail. 

2.  Gypsum :  Can  be  scratched  by  the  thumbnail. 

3.  Calcite  :  Not  scratched  by  the  thumbnail,  but  easily  cut  with  a  knife. 

4.  Fluorite  :  Can  be  cut  with  a  knife,  but  less  easily  than  calcite. 

5.  Apatite :  Can  be  cut  with  a  knife,  but  only  with  difficulty. 

6.  Orthoclase  feldspar :  Can  be  cut  with  a  knife  only  with  great  difficulty  and 

on  thin  edges. 

7.  Quartz :  Cannot  be  cut  with  a  knife  ;  scratches  glass. 

8.  Topaz :  Will  scratch  quartz. 

9.  Corundum :  Will  scratch  topaz. 
10.    Diamond :  Will  scratch  corundum. 


QUARTZ  13 

quartz  porphyry,  liparite,  gneiss,  mica  schist,  quartzite,  and 
sandstones.  In  the  granites,  gneisses,  and  schists  it  occurs  in 
the  form  of  irregular  granules  destitute  of  crystal  outlines. 
In  the  quartz  porphyries  and  liparites  it  is  found  as  a  porphy- 
ritic  constituent,  usually  with  well-defined  crystal  outlines, 
which  may  however  have  become  more  or  less  obliterated 
through  the  corrosive  action  of  a  molten  magma.  (See  Fig.  3, 
PI.  5.)  In  the  secondary  rocks,  quartzite  and  sandstone,  the 
quartz  occurs  as  more  or  less  rounded  or  irregularly  angular 
grains  without  crystal  outlines,  except  it  may  be  through  a 
secondary  deposition  of  silica,  as  explained  on  p.  158.  Quartz 
is  the  hardest  and  most  indestructible  of  the  common  constitu- 
ents, and  hence  when  rocks  containing  it  decompose  and  their 
debris  becomes  exposed  to  combined  chemical  and  mechanical 
agencies,  it  remains  unaltered  to  the  very  last,  forming  the 
chief  constituent  of  beds  of  sand  and  gravel,  which  in  turn 
may  become  transformed  into  sandstones,  quartzites,  or  con- 
glomerates. 

Quartz  is  usually  easily  recognized,  either  under  the  micro- 
scope or  by  the  unaided  eye,  by  its  clear,  colorless  appear- 
ance, irregular,  glass-like  fracture,  —  having  no  true  cleavage, 
—  hardness,  and  insolubility  in  any  acids  but  hydrofluoric. 
Under  the  microscope  it  appears  in  clear,  pellucid  grains,  often 
highly  charged  with  minute  cavities  filled  with  liquid  and 
gaseous  carbonic  acid,  the  latter  like  the  bubble  in  a  spirit 
level,  dancing  about  from  side  to  side  of  its  minute  chamber  as 
though  endowed  with  life.  In  other  cases  the  cavity  may  be 
filled  with  a  saline  solution  from  which  has  separated  out  a 
minute  cube  of  common  salt. 

As  a  secondary  constituent  quartz  occurs,  filling  veins  and 
cracks  in  other  rocks,  and  in  the  impure  crypto-crystalline  and 
amorphous  forms  known  as  chalcedony,  chert,  flint,  opal,  hya- 
lite, and  agate  is  found  as  an  infiltration  product  in  the  cavities 
of  many  trappean  rocks,  in  lenticular  and  oval  concretionary 
masses  in  limestones,  and  replacing  the  organic  matter  of  wood 
and  other  organisms.  The  name  tridymite  is  given  to  a  quartz 
occurring  in  minute,  usually  microscopic,  tablets  in  cavities  in 
volcanic  rocks,  particularly  the  more  acid  varieties.  (See  fur- 
ther on  p.  71.) 

The  Feldspars.  —  Hardness,  5  to  7;  specific  gravity,  2.5  to 
2.8.  The  feldspars  are  essentially  anhydrous  silicates  of  alu- 


14  THE    MINERALS   CONSTITUTING  KOCKS 

minum,  with  varying  amounts  of  lime,  potash,  or  soda,  and 
rarely  barium.  They  have  in  common  the  characteristics  of 
two  easy  cleavages  inclined  to  one  another  at  an  angle  of 
90°,  or  nearly  90°;  close  relationship  in  optical  properties; 
similarity  in  colors,  which  vary  from  clear  and  transparent 
through  white,  yellowish  pink,  and  red;  more  rarely  greenish, 
and  often  opaque  through  impurities  or  decomposition;  and 
lastly,  a  constant  intergradation  in  composition,  as  already 
noted  on  p.  9. 

Nine  varieties  of  feldspar  are  commonly  recognized,  which 
on  crystallographic  grounds  are  divided  into  two  groups:  the 
first,  crystallizing  in  the  monoclinic  system,  including  ortho- 
clase  and  hyalophane;  and  the  second,  crystallizing  in  the 
triclinic  system,  including  microcline,  anorthoclase,  and  the 
albite-anorthite  series  albite,  oligoclase,  andesine,  labradorite, 
and  anorthite. 

The  Monoclinic  Feldspars :  Orthoclase  (Sanidin)^  Potash  Feld- 
spars. —  Composition :  K2Al2Si6O16  =  silicia,  64.7  % ;  alumina, 
18.4%;  potash,  16.9%. 

This  is  one  of  the  commonest  and  most  abundant  of  feldspars, 
and  forms  an  essential  constituent  of  the  acid  rocks,  such  as  gran- 
ite, gneiss,  syenite,  and  the  orthoclase  and  quartzose  porphyries; 
more  rarely  it  occurs  as  an  accessory  in  the  more  basic  erup- 
tives.  Under  the  name  sanidin  is  included  the  clear  glassy 
variety  of  orthoclase  occurring  in  tertiary  and  modern  lavas, 
such  as  trachyte,  phonolite,  and  the  liparites. 

Among  the  older  rocks  orthoclase  not  infrequently  occurs  in 
very  coarse  pegmatitic  crystallizations  with  quartz  and  mica, 
and  is  quarried  for  utilization  in  pottery  manufacture.  As  a 
rock  constituent  the  potash  feldspars  are  of  primary  impor- 
tance, imparting  by  their  preponderance,  not  merely  color 
and  important  structural  features,  but  on  their  decomposition 
yielding  up  the  alkali  potash,  valuable  for  plant  food,  and  the 
mineral  kaolin  so  essential  for  porcelain  ware,  or  in  its  impure 
state,  as  clay  for  pottery  and  brick  making.  In  the  thin  sec- 
tions, under  the  microscope,  the  orthoclase  of  the  older  rocks 
is,  as  a  rule,  found  to  be  quite  opaque,  or  at  least  muddy, 
through  impurities  or  incipient  kaoliiiization.  In  many  erup- 
tives  it  has  been  one  of  the  first  minerals  to  separate  out  from 
the  molten  magma,  and  shows,  therefore,  more  or  less  well- 
defined  crystallographic  boundaries — is  idiomorphic,  to  use  a 


THE   TRICLINIC   FELDSPARS 


15 


more  technical  term.  A  well-defined  zonal  structure  is  fre- 
quently observed,  which  is  due  to  interrupted  periods  of 
growth,  and  not  infrequently  to  a  gradual  change  in  the  char- 
acter of  the  magma,  whereby  the  outer  zones  are  more  or  less 
translucent  or  opaque  from  impurities.  Twin  structure  is  very 
common  after  what  is  known  as  the  Carlsbad  law,  and  when 
the  crystals  are  of  sufficient  size  is  easily  recognized  by  the 
unequal  reflection  of  the  light  from  the  two  sides  of  a  crystal 
on  a  cleavage  surface. 

The  Triclinic  Feldspars.  —  The  chemical  relationship  exist- 
ing between  the  triclinic  feldspars  is  shown  in  the  following 
table:  — 


SiO2 

A1203 

K20 

Na20 

CaO 

Microcline  

65.00% 

18.00% 

17.00% 

Albite 

68  00 

20  00 

12  00  °/ 

^^^^\^^^ 

Olio-oclase 

62.00 

24.00 

9  00 

™aJIUL)  h 

5  00 

Labradorite 

53.00 

30.00 

4  00 

13  00 

Anorthite    .              ... 

43.00 

37.00 

20  00 

Considering  only  the  last  four  of  these,  as  arranged,  it  will 
be  noted  that  they  become  gradually  poorer  in  the  acid  element 
silicia,  and  richer  in  alumina  and  other  bases;  that  is,  they 
become  more  basic.  Also  that  albite  carries  some  12  %  of  soda 
and  no  lime;  that  oligoclase  carries  9  %  of  soda  and  5  %  of  lime; 
labradorite  but  4  %  of  soda  and  13  %  of  lime,  while  anorthite, 
the  most  basic  of  all,  has  no  soda,  and  carries  20  %  of  lime. 
They  have  hence  come  to  be  known,  respectively,  as  soda  feld- 
spar, soda-lime  feldspar,  lime-soda  feldspar,  and  lime  feldspar. 
As  a  matter  of  fact,  however,  these  varieties  all  grade  into  one 
another,  through  the  replacing  power  of  the  various  elements, 
and  are  regarded,  not  as  true  species,  but  rather  as  isomorphous 
admixtures,  forming  what  is  known  as  the  albite-anorthite 
series. 

Their  distinction,  either  in  hand  specimens  by  the  unaided 
eye,  or  in  thin  sections  by  the  microscope,  is  a  matter  of  con- 
siderable difficulty,  and  as  in  addition  to  other  characteris- 
tics they  have  in  common  two  eminent  cleavages  occurring  at 
oblique  angles,  it  has  become  customary  to  group  all  under 
the  general  term  of  plagioclase,  a  name  derived  from  two 


16  THE   MINERALS   CONSTITUTING  ROCKS 

Greek  words  signifying  oblique  and  fracture.  We  can  then 
treat  of  the  subject  under  the  heads  of  (1)  microline  and 
(2)  plagioclase. 

(1)  Microcline  (Triclinic  Potash  Feldspar).  —  As  a  rock  con- 
stituent,  this  feldspar  is  in  every  way  nearly,   if  not  quite, 
identical  with  orthoclase,  from  which  it  can  be  distinguished 
only  in  thin  sections  under  the  microscope.     Its  composition, 
manner  of  occurrence,  and  associations  are  those  of  orthoclase, 
and   need   not  be  repeated  here.     Anorthoclase  is  a  triclinic 
soda-potash  feldspar  of  a  form  closely  resembling  that  of  ortho- 
clase and  which  for  all  present  purposes  may  be  regarded  as 
orthoclase  in  which  soda  replaces  a  considerable  proportion  of 
the  potash. 

(2)  The  Plagioclases.  —  With  the  exception   of   albite   the 
plagioclases   are  all   prominent   and   essential   constituents  of 
the  basic  eruptives.     As  a  rule  they  are  recognizable  only  as 
fSldsJTars  by  the  unaided  eye,   and  recourse  must  be  had  to 
the  microscope  or  to  chemical  tests  for  their  final  determina- 
tion.    Examined  in  thin  sections  and  by  polarized  light,  they 
almost  invariably  show  a  beautiful  parallel  banding  in   light 
and  dark  colors,  which  is  due  to  multiple  twinning,  the  alter- 
nate bands  becoming  light  and  dark  in  turn  as  the  stage  of  the 
microscope  is  revolved.      When  the  crystals  are  of  sufficient 
size,  this  twinning  is  sometimes  evident  in  the  form  of   fine 
straight,  parallel  bands,  or  strise,  but  in  rock  masses,  as  already 
noted,  recourse  must  be  made  to  microscopic  methods.     In  form 
the  plagioclase  of  effusive  rocks  is  most  frequently  slender  and 
elongated,  lath-shaped,  as  commonly  described,  and  often  with 
very  perfect  crystal  outlines.     In  the  norites  and  gabbros.  they 
are  often  short  and  stout,  imparting  a  granular  character  to  the 
rock.     They  occur  frequently  in  crystals  of  two  or  more  gen- 
erations, of  which  the  earlier  formed  are  usually  the  largest 
and  best  developed.     The  common  forms  are  described  in  de- 
tail below :  — 

(1)  Albite,  or  soda  feldspar,  occurs  as  an  original  constituent 
in  many  granites  in  company  with  orthoclase;  it  is  also  found 
in  gneiss,  the  crystalline  schists,  and  not  infrequently  in  diorite, 
phonolite,  trachyte,  and  other  eruptives.  (2)  Oligoclase,  a  soda- 
lime  feldspar,  occurs  like  albite  in  the  acid  eruptives  like  gran- 
ite and  quartz  porphyry,  but  is  also  a  common  constituent  of 
diorite,  and  the  younger  eruptives  such  as  trachyte,  the  ande- 


THE   TRICLINIC   FELDSPARS  17 

sites,  and  more  rarely  of  the  diabases.  It  is  also  a  constituent 
of  many  gneisses.  (3)  Labradorite,  or  lime-soda  feldspar,  is  a 
prominent  constituent  of  the  basic  eruptives  of  all  geologi- 
cal ages,  such  as  the  norites,  diabases,  diorites,  and  basalts. 
Andesine  and  bytownite  are  closely  allied  varieties  of  similar 
'habit,  the  first  being  a  trifle  more  acid,  and  the  second  more 
basic  than  labradorite.  (4)  Anorthite,  or  lime  feldspar,  is 
also  a  prominent  and  important  constituent  of  the  basic 
eruptives,  and  has  been  found  in  meteorites  and  terrestrial 
peridotites. 

On  account  of  their  abundance  and  wide  distribution,  as  well 
as  on  account  of  the  character  of  their  decomposition  products, 
the  feldspars  are  to  be  considered  as  among  the  most  important 
of  rock  constituents.  As  it  is  from  the  debris  of  the  older 
feldspathic  rocks  that  have  been  made  up  a  large  proportion 
of  all  the  sedimentaries  of  more  recent  date,  so  too  it  may  be 
claimed  that  from  the  decomposition  of  this  feldspathic  con- 
stituent has  been  derived  a  large  share  of  the  salts  of  potash, 
lime,  and  soda,  as  well  as  aluminous  silicates  which  form  so 
essential  a  portion  of  the  soils.  The  method  of  feldspathic 
decomposition  as  commonly  understood  is  given  on  p.  237. 

This  decomposition  usually  manifests  itself  by  a  whitening 
of  the  mass,  accompanied  by  opacity  and  a  general  softening, 
whereby  it  falls  away  to  loose  powder  unless  confined.  As  seen 
in  thin  sections  under  the  microscope,  the  decomposition  goes 
on  most  rapidly  along  lines  of  cleavage,  naturally  attacking  the 
outer  portions  first,  so  that  the  crystals  show  fresh  unaltered 
cores  surrounded  by  opaque  and  "  muddy  "  borders.  In  cases 
where  the  feldspars  carry  iron  this  usually  makes  its  presence 
known  by  a  reddening  or  browning  of  the  mass,  due  to  oxida- 
tion. In  presence  of  abundant  carbonic  acid,  the  liberated  iron 
may  enter  into  combination  as  a  carbonate  and  the  color  remain 
unchanged. 

Daubree,  who  submitted  feldspathic  fragments  to  trituration 
in  revolving  cylinders  of  stone  and  iron,  found  that  in  all  such 
cases  not  merely  were  the  particles  worn  down  to  the  condi- 
tion of  fine  silt,  but  that  there  was  an  actual  decomposition, 
whereby  a  certain  proportion  of  the  alkalies  in  the  form  of 
soluble  silicates  were  formed  in  the  water  with  which  the  cyl- 
inders were  partially  filled.  When  the  trituration  was  carried 
on  in  iron  cylinders,  a  certain  amount  of  iron  oxides  were 


18  THE   MINERALS   CONSTITUTING  ROCKS 

formed  which  combined  with  the  silica  of  the  alkaline  silicate, 
leaving  the  alkali  itself  free.  As  in  nearly  all  decomposing 
rocks  there  exists  more  or  less  of  iron  oxides  from  decomposing 
ferruginous  minerals,  it  is  not  impossible  that  a  similar  reaction 
is  there  going  on. 

The  production  of  kaolin  through  feldspathic  decomposition 
has  become  so  well  recognized  that  it  is  customary  to  speak 
of  this  form  of  decomposition  as  kaolinization,  a  term  which  we 
shall  have  frequent  cause  to  use  as  we  proceed. 

It  should  be  noted  that  orthoclase,  though  so  frequently 
found  muddied  and  impure,  apparently  in  an  advanced  stage 
of  decomposition,  does  not  in  reality  decompose  so  readily  as 
the  plagioclase  (soda-lime)  varieties.  This  fact  has  been  noted 
by  Lemberg,1  who  states  that  the  apparent  decomposition  may 
be  due  to  physical  causes,  as  disintegration,  inclusions  of  some 
easily  decomposable  silicate,  or  to  originally  water-filled  cavities 
whose  contents  have  been  absorbed  through  the  formation  of 
secondary  hydrous  silicates. 

Leucite.  —  Composition:  Silica,  55.0  %  ;  alumina, 23.5  %  ;  pot- 
ash, 21.5%. 

Leucite  occurs  as  an  original  and  essential  constituent  of 
many  volcanic  rocks,  such  as  leucitophyre,  leucotephrite,  and 
leucitite.  More  rarely  it  occurs  in  trachyte.  It  is  a  common 
associate  of  nepheline  in  recent  lavas,  and  has  been  found  asso- 
ciated with  elseolite  in  the  elseolite  syenites  of  Hot  Springs, 
Arkansas.  When  well  developed  it  shows  polyhedral,  garnet- 
like  outlines. 

Leucite  as  a  rock  constituent  is  not  an  abundant  mineral 
except  in  rare  instances.  Its  chief  interest,  from  our  present 
standpoint,  lies  in  its  high  percentage  of  potash  which  must 
become  available  as  plant  food  on  decomposition.  Leucite  is 
a  common  constituent  of  certain  Vesuvian  lavas,  and  it  is  not 
improbable  that  this  fact  may  account  in  part  for  the  well- 
known  fertility  of  the  soils  of  this  region,  though  naturally 
climatic  influence  has  much  to  do. 

Nepheline  (Elaeolite).  —  These  names  are  given  to  what  are 
varietal  forms  of  one  and  the  same  mineral.  In  composition 
they  are  silicates  of  alumina,  soda,  and  potash  of  the  formula 
(NaK)2Al2Si2O8  =  silica,  41.24;  alumina,  35.26;  potash,  6.46; 
soda,  17.04. 

i  Zeit.  Deut.  Geol.  Gesellschaft,  35,  1883. 


THE   AMPHIBOLES  19 

Nepheline  occurs  in  Tertiary  and  post-Tertiary  eruptive  rocks, 
and  is  an  essential  constituent  of  phonolite,  tephrite,  and  nephe- 
linite.  Secondary  nepheline  has  been  found  in  the  ejected  vol- 
canic blocks  found  in  the  lava  of  Mount  Somma.  The  variety 
elseolite  occurs  only  in  older  rocks,  and  is  an  essential  constitu- 
ent of  elseolite  syenite.  Cancrinite  is  a  yellowish  granular 
mineral,  in  some  cases  apparently  resulting  from  the  alteration 
of  ekeolite,  with  which  it  occurs. 

Both  nepheline  and  elseolite  gelatinize  readily  with  hydro- 
chloric acid,  and  the  powdered  rock  when  treated  on  a  glass  slide 
with  this  acid  yields  abundant  microscopic  cubes  of  sodium 
chloride.  This  is  one  of  the  easiest  of  microchemical  tests  for 
the  determination  of  the  mineral.  Nepheline  occurs  as  a  rule 
in  well-defined  short  and  stout  hexagonal  prisms,  which  in 
longitudinal  sections  show  up  as  short,  colorless  rectangular 
areas  extinguishing  parallel  with  the  sides  of  the  prism.  Elseo- 
lite differs  in  being  more  opaque  and  occurring  in  less  well- 
defined,  more  granular  forms.  When  occurring  in  sufficient 
abundance  in  a  rock  mass  it  is  readily  recognized  by  its  char- 
acteristic greasy  appearance.  The  mineral  undergoes  a  ready 
alteration,  giving  rise  to  zeolitic  minerals  and  on  ultimate 
decomposition  through  weathering,  yielding  a  rich  and  fertile 
soil. 

The  Amphiboles.  —  Composition :  Two  principal  varieties  are 
recognized.  (1)  Non-aluminous,  consisting  mainly  of  the 
meta-silicates  of  magnesium  and  calcium,  with  55  to  59  %  of 
silica,  21  to  27  %  of  magnesia,  11  to  15  %  of  lime,  and  small  pro- 
portions of  protoxides  of  iron  and  manganese.  Under  this  head 
are  included  the  white,  gray,  and  pale  green,  often  fibrous  forms, 
as  tremolite,  actinolite,  and  asbestos.  (2)  Aluminous,  contain- 
ing silica,  40  to  51  % ;  magnesia,  10  to  23  % ;  alumina,  6  to  14  % ; 
lime,  10  to  13%;  ferrous  and  ferric  oxides,  12  to  20%.  Here 
are  included  the  dark  green,  brown,  and  black  varieties. 

The  aluminous  variety,  common  hornblende,  is  an  original 
and  essential  constituent  of  diorite,  and  of  many  varieties  of 
granite,  gneiss,  syenite,  schist,  andesite,  and  trachyte,  and  is 
also  present  as  a  secondary  constituent  in  many  rocks,  result- 
ing from  the  molecular  alteration  of  the  augite.  The  non- 
aluminous  varieties  occur  in  gneiss,  crystalline  limestone,  and 
other  metamorphic  rocks. 

By  the  unaided  eye,  or  by  means  of  blowpipe  tests,  it  is  often 


20 


THE   MINERALS   CONSTITUTING   ROCKS 


impossible  to  distinguish  the  minerals  of  this  group  from 
the  pyroxenes.  In  the  thin  sections  this  distinction  is,  however, 
a  matter  of  comparative  ease,  basal  sections  showing  not  merely 
a  greater  development  of  prismatic  faces,  but  also  cleavages 
cutting  at  angles  of  66°  and  124°  instead  of  nearly  at  right 
angles,  as  in  the  latter.  Green  fibrous  hornblendes  frequently 
result  from  the  molecular  alteration  of  augite,  and  all  varieties 
are  susceptible  of  alteration  into  chloritic  and  ferruginous 
products  with  the  separation  of  calcite.  In  the  recent  lavas  it 
is  a  common  occurrence  to  find  the  hornblendes  surrounded  by 
a  black  border,  or  wholly  changed  by  corrosion  of  the  molten 
magma  into  an  aggregate  of  small  black  opaque  granules,  which 
in  certain  instances  have  been  proven  to  be  augites. 

On  decomposing,  the  amphiboles  give  rise  to  ferruginous  and 
aluminous  or  magnesian  products,  as  do  the  pyroxenes,  next  to 
be  described.  With  the  darker  colored  varieties,  the  decompo- 
sition begins  with  hydration  and  the  peroxidation  of  the  iron 
along  lines  of  cleavage  and  fracture,  whereby  the  crystal 
becomes  riddled  with  corroded  areas  filled  with  the  liberated 
iron  in  the  form  of  hydrated  sesquioxide. 

When  the  disintegration  is  complete,  the  whole  mass  is  con- 
verted into  an  ochre-brown,  earthy  substance.  These  chemical 
changes  are  indicated  in  the  following  analysis  of  I.  fresh,  and 
II.  decomposed  hornblende  from  Haavi  on  Fillef  jeld,  Norway : 1 — 


I 

II 

Silica    .          

4537 

4032 

Alumina    

1481 

1749 

Iron  protoxide    .... 
Manganese 

8.74 
1  50 

Iron  peroxide     .     .     . 

18.26 
2  14 

Lime 

1491 

537 

Magnesia  

1433 

923 

Water  

800 

99.66 

100.81 

The  most  striking  features  of  the  above  analyses  are  (1) 
the  complete  conversion  of  the  protoxides  into  sesquioxides, 
(2)  the  loss  in  lime  and  magnesia  which  have  presumably 


1  Bischof's  Chemical  Geology,  Vol.  II,  p.  354. 


THE   MONOCLINIC   PYROXINES  21 

been  carried  away  in  the  form  of  carbonates,  and  (3)  the 
assumption  of  8  %  of  water.  As  the  dark  aluminous  and 
ferruginous  hornblendes  are  among  the  commonest  and  most 
wide-spread  of  minerals,  it  is  apparent  from  the  above  that 
they  may  have  an  important  bearing  upon  the  color  and  physi- 
cal qualities  of  the  residual  clays ;  to  which  they  thus  give 
rise.  The  peroxidation  of  the  iron  gives  yellow,  brown,  or  red 
colors,  while  the  hydrated  aluminous  silicate  (clay)  imparts 
tenacity.  The  final  product  of  such  decomposition  is,  then,  a 
ferruginous  clay. 

The  Pyroxenes.  —  The  rock-forming  pyroxenes  are  divided 
upon  crystallographic  grounds  into  two  groups,  the  one  ortho- 
rhombic  in  crystallization,  and  the  other  monoclinic.  All  varie- 
ties, when  in  good  crystalline  form,  show  in  basal  sections  an 
octagonal  outline  bounded  by  prismatic  and  pinacoidal  faces 
and  with  a  well-defined  cleavage  parallel  with  the  prism  faces. 
Chemically  they  are  silicates  of  magnesia  and  iron  with  lime 
and  alumina  in  varying  proportions.  They  are  hard,  tough 
minerals  and  have  an  important  bearing  upon  the  physical 
properties  of  the  rocks  of  which  they  form  a  part.  Their  dis- 
tribution, in  some  of  their  varieties,  is  almost  universal,  being 
found  in  metamorphic  and  eruptive  rocks  of  almost  every  class 
and  every  age. 

The  Monoclinic  Pyroxenes.  — Two  principal  varieties  are  recog- 
nized. (1)  Pyroxenes  containing  little  or  no  alumina,  and  com- 
posed of  silica,  45.95  to  55.6  %  ;  lime,  21.06  to  25.9  % ;  magnesia, 
13.08  to  18.5  %,  with  sometimes  varying  quantities  of  iron  oxides 
and  water.  Under  this  head  are  included  the  lighter  colored 
varieties,  malacolite,  sahlite,  and  diallage.  (2)  Pyroxenes  con- 
taining alumina,  and  composed  of  silica,  49.40  to  51.50  %;  alu- 
mina, 6.15  to  6.70%;  magnesia,  13.06  to  17.69%;  lime,  21.88 
to  23.80%;  iron  oxides,  0.35  to  7.83%,  with  sometimes  small 
quantities  of  soda  and  water.  Under  this  head  are  included 
the  darker  varieties,  augite  and  leucaugite. 

The  lighter  colored,  non-aluminous  varieties,  malacolite  and 
sahlite,  are  common  in  mica  and  hornblendic  schists,  gneiss, 
and  granite,  though  not  always  in  sufficient  abundance  to  be 
noticeable  to  the  naked  eye.  The  foliated  variety,  diallage, 
is  an  essential  constituent  of  the  rock  gabbro,  and  is  also 
common  in  peridotites.  The  darker  colored,  aluminous  vari- 
ety, augite,  is  an  essential  constituent  of  diabase  and  basalt, 


22  THE   MINERALS   CONSTITUTING   ROCKS 

and  also  occurs  in  many  syenites,  andesites,  and  other  eruptive 
rocks. 

In  the  thin  sections  the  monoclinic  pyroxenes  are  usually 
readily  recognized  by  their  nearly  rectangular  cleavages  on 
basal  sections  (see  Fig.  1),  lack  of  pleochroism,  and  high 
extinction  angles  on  sections  parallel  to  the  clinopinacoids. 
The  aluminous  varieties  undergo  alteration  into  chloritic  and 
ferruginous  products,  while  the  non-aluminous  give  rise  to  ser- 
pentine, either  process  being  attended  by  the  separation  of 
free  calcite. 

JEgerine  and  acmite  are  soda-bearing  pyroxenes  corre- 
sponding to  the  formula  Na2OFe2O34SiO2.  They  are  less 
abundant  than  the  above-mentioned  varieties,  and  so  far  as 
yet  described  seem  to  be  confined  mainly  to  the  elseolite 
syenites. 

The  OrthorJiombic  Pyroxenes.  —  These  are  essentially  silicates 
of  magnesia  and  iron,  the  latter  replacing  the  former  in  varying 
proportions  up  to  as  high  as  25  %.  Two  principal  varieties  are 
recognized,  the  distinction  being  founded  mainly  upon  their 
optical  properties  which  seem  to  be  affected  very  largely  by  the 
percentages  of  iron.  Enstatite  is  the  theoretically  pure  mag- 
nesian  silicate  of  the  formula  MgSiO3,  but  which,  as  a  matter 
of  fact,  usually  contains  from  2  to  10  %  or  more  of  iron.  The 
highly  ferruginous  varieties  are  known  as  bronzite,  from  their 
bronze-like  lustre.  Hypersthene  differs  from  enstatite  in  being 
strongly  pleochroic  in  thin  sections,  and  it  contains  from  10  to 
25  %  of  ferruginous  oxide. 

Both  enstatite  and  hypersthene  are  common  constituents  of 
basic  igneous  rocks,  such  as  the  gabbros,  norites,  and  perido- 
tites.  Enstatite  is  a  common  constituent  of  meteorites,  occur- 
ring not  infrequently  in  peculiar  fan-shaped,  radiating  masses 
not  greatly  unlike  certain  organic  forms  for  which  they  were 
once  mistaken.  Both  forms  are  liable  to  alteration,  giving  rise 
to  serpentinous  pseudomorphs  to  which  the  name  bastite  has 
been  given,  and  to  talcose  and  chloritic  products.  The  general 
character  of  the  decomposition  products  of  the  pyroxenes,  as 
well  as  the  methods  by  which  the  decomposition  progresses, 
are  in  every  way  similar  to  those  of  the  amphiboles,  and  need 
not  be  further  dwelt  upon  here. 

The  Micas.  —  There  are  several  species  of  mica  which  are 
prominent  as  rock  constituents,  the  more  important  being  the 


THE   MICAS  23 

white  variety,  muscovite,  and  the  dark  variety,  biotite.  Both 
occur,  as  a  rule,  in  thin,  platy  forms,  splitting  readily  into  thin, 
elastic  folia,  which  in  crystalline  form  are  hexagonal  in  outline. 
The  folia  are  often  bent  and  distorted,  and  the  mineral  not 
infrequently  undergoes  alteration  into  a  chloritic  or  sericitic 
product.  The  micas  exercise  an  important  influence  upon  the 
rocks  containing  them,  on  both  color  and  structural  grounds. 
Other  things  being  equal,  the  muscovite-bearing  rocks  are 
lighter  in  color  than  those  carrying  biotite.  If  the  mica  plates 
are  arranged  in  definite  planes,  the  rock  assumes  a  schistose 
structure  and  splits  more  or  less  readily  into  sheets  —  an  impor- 
tant feature  from  an  economic  standpoint.  Muscovite,  or 
potash  mica,  a  silicate  of  alumina  and  potash,  is  a  constituent 
of  many  granites,  gneisses,  and  schists,  but  is  rarely  met  with 
in  other  rocks,  and  is  wholly  wanting  in  the  basic  eruptives. 
Another  white  or  nearly  colorless  mica  is  sericite,  a  silvery 
white,  or  greenish,  hydrous,  secondary  constituent  of  metamor- 
phic  schists,  or  occurring  as  an  alteration  product  from  feldspar : 
paragonite  and  margarite  are  other  hydrous  micas,  confined 
mainly  to  the  schists  and  to  veins.  Lepidolite,  a  lithia  mica  of 
a  white  or  faint  pink  color,  is  frequently  found  in  pegmatitic 
veins  in  the  older  rocks. 

Biotite,  the  black  iron  mica,  is  a  silicate  of  alumina,  iron,  and 
magnesia,  and  is  much  more  general  in  its  distribution  than  is 
muscovite,  occurring  in  both  eruptive  and  metamorphic  rocks 
of  all  kinds  and  of  all  ages.  It  undergoes  alteration  into 
chloritic  and  ferruginous  products  and  is  often  an  impor- 
tant feature  in  hastening  rock  disintegration.  Other  black 
micas,  sometimes  distinguishable  from  biotite  only  by  chemi- 
cal means,  are  lepidomelaiie  and  houghtonite.  A  pearl  gray 
potash  mica  phlogopite  is  an  important  constituent  of  many 
limestones,  as  in  northern  New  York  and  adjacent  portions  of 
Canada. 

All  micas,  owing  to  their  eminently  fissile  structure,  allow  the 
ready  percolation  of  moisture,  and  hence,  though  in  themselves 
of  difficult  solubility,  are  elements  of  weakness  in  any  stone 
of  which  they  may  form  a  part.  The  characteristic  form  of 
decomposition  begins  as  in  other  silicate  minerals,  with  hydra- 
tion.  This  in  the  dark  varieties  is  accompanied  by  a  higher 
oxidation  of  the  iron.  The  foliie  gradually  lose  their  elasticity 
and  crumble  away,  the  bases  being  removed  in  solution  as 


24 


THE    MINERALS   CONSTITUTING   ROCKS 


before.  The  complete  decomposition  of  the  micas  is,  however, 
brought  about  very  slowly,  and  almost  any  granitic  soil,  how- 
ever thoroughly  decomposed,  will,  on  washing,  show  small  flakes 
of  the  mineral  still  remaining.  However  rusty,  too,  these  may 
appear,  a  little  hydrochloric  acid  cleans  them  up,  showing  rem- 
nant shreds  still  fresh  and  readily  recognizable.  For  some 
unexplained  reason  those  granitic  rocks  containing  a  consider- 
able proportion  of  white  mica  are  almost  invariably  more  friable 
and  easily  disintegrated  than  those  containing  biotite. 

Olivine  (Chrysolite,  Peridote). —  Composition:  Silicate  of  iron 
and  magnesia,  (MgFe)2  SiO4. 

This  is  an  essential  constituent  of  basalt,  dunite,  limburgite, 
Iherzolite,  and  pikrite,  and  a  prominent  ingredient  of  many 
lavas,  diabases,  gabbros,  and  other  igneous  rocks.  It  also  occurs 
occasionally  in  metamorphic  rocks  and  is  a  constituent  of  most 
meteorites.  Olivine  is  subject  to  extensive  alteration,  becom- 
ing changed  by  hydration  into  serpentine  or  talcose  and  chloritic 
products,  with  the  separation  of  free  iron  oxides.  Under  the 
microscope  olivine  is  as  a  rule  easily  recognized  by  its  lack  of 
cleavage  and  brilliant  polarization  colors.  It  occurs  in  well- 
defined  crystals  and  also  in  irregular  grains,  either  singly  or 
grouped  in  peculiar  clusters  to  which  the  name  poly  somatic  has 
been  applied  by  Tschermak.  The  serpentinous  alteration  takes 
place  along  the  irregular  curvilinear  lines  of  fracture,  and  under 
favorable  conditions  continues  until  the  transformation  is  com- 
plete. The  following  analyses  by  Holland,  as  quoted  in  Teall's 
British  Petrography,  illustrate  the  simplicity  of  the  chemical 
changes  which  here  take  place :  — 


I 

II 

III 

Si02 

41  32  °/ 

42  72  °/ 

43  48  °/ 

Alo03 

•XL.VtJ       /Q 

028 

«*«**!    /O 

006 

Fe203       ...... 

239 

225 

CrO    

005 

Trace 

M^O 

5469 

42  52 

43  48 

H2O         .          .     . 

020 

1339 

13  04 

98.93  % 

100.94  % 

100.00  % 

I.    Olivine,    Snarum,   Norway.      II.    Serpentine    derived    from    the    same. 
III.  The  theoretical  composition  of  serpentine. 


EPIDOTE   AND   ALLANITE  25 

Aside  from  the  assumption  of  some  13  %  of  water,  the  princi- 
pal change,  as  will  be  noted,  is  a  loss  in  magnesia  which  as  a 
rule  separates  out  as  a  carbonate.  The  iron,  which  existed  as 
protoxide,  is  further  oxidized  and  crystallizes  out  along  lines  of 
fracture  as  magnetite  or  hematite,  or  in  the  hydrous  sesquioxide 
form  known  as  limonite.  Through  decomposition,  a  portion  or 
all  of  the  silica  may  be  set  free  as  opal  or.  chalcedony,  the  mag- 
nesia going 'over  to  the  condition  of  carbonate,  and  the  iron 
passing  into  various  hydrated  oxide  forms  such  as  are  most 
stable  under  the  existing  circumstances. 

Epidote.  —  Composition:  Silica,  37.83%;  alumina,  22.63%; 
iron  oxides,  15.98  %  ;  lime  23.27  %  ;  water  2.05  %. 

This  mineral  is  a  common  constituent  of  many  granites, 
gneisses,  and  schists,  especially  the  hornblendic  varieties.  It 
is  particularly  abundant,  however,  as  a  secondary  constituent 
in  basic  eruptives,  where  it  results  from  the  alteration  of  the 
original  ferromagnesian  constituents  such  as  the  augites,  horn- 
blendes, or  micas.  It  is  the  presence  of  this  mineral  or  a  sec- 
ondary chlorite  that  gives  the  characteristic  color  to  many  of 
the  so-called  greenstones  (altered  basalts,  diabases,  diorites,  etc.). 

The  name  piedmontite  is  given  to  a  red  manganese  epidote, 
which  has  been  found  in  certain  Japanese  schists  and  has  also, 
in  sparing  amounts,  been  observed  by  Professor  Ha  worth,1  in  the 
quartz  porphyries  of  Missouri,  and  a  few  foreign  porphyrites. 
Zoisite  is  a  closely  related  mineral  crystallizing  in  the  ortho- 
rhombic  system  and  relatively  poorer  in  iron  and  richer  in 
alumina  than  is  epidote.  It  is  chiefly  characteristic  of  the  crys- 
talline schists,  though  sometimes  found  in  granitic  rocks,  inter- 
grown  with  common  epidote  as  has  been  noted  in  Maryland,  by 
Keyes.2 

Allanite,  or  orthite,  as  it  is  sometimes  called,  is  closely  allied 
to  common  epidote,  but  contains  cerium  and  other  of  the  more 
rare  alkaline  earths.  In  the  form  of  brown  acicular  crystals  it 
is  a  common  constituent  of  New  England  granites  and  has 
recently  been  described  in  a  granite  porphyry  near  Ilchester, 
Maryland,  where  it  occurs  enclosed  as  a  nucleus  in  the  ordinary 
epidote. 

Calcite  (Calcium  Carbonate).  —  Composition:  CaCO3  =  Car- 
bon dioxide,  44  %  ;  lime,  56  %.  Hardness,  3. 

1  American  Geologist,  Vol.  I,  p.  365. 

2  15th  Ann.  Rep.  U.  S.  Geol.  Survey,  1890-94. 


26  THE   MINERALS   CONSTITUTING   ROCKS 

This  is  an  original  constituent  of  many  secondary  rocks, 
such  as  limestone,  ophiolite,  and  calcareous  shales.  It  is  the 
essential  constituent  of  most  marbles,  of  stalactites,  travertine, 
and  calc-sinter.  The  shells  of  foraminifera,  brachiopods,  crus- 
taceans, and  many  lamellibranchs  and  gasteropods  are  also  of 
this  material.  As  a  secondary  constituent,  resulting  from  the 
decomposition  of  other  minerals,  it  occurs  almost  universally, 
filling  wholly  or  in  part  cavities  in  rocks  of  all  ages,  such  as 
granite,  gneiss,  syenite,  diabase,  diorite,  liparite,  trachyte, 
andesite,  and  basalt. 

The  effervescence  of  the  mineral  when  treated  with  a  dilute 
acid  furnishes  the  most  ready  means  for  its  detection.  Under 
the  microscope  it  appears  as  colorless  grains  with  faint  irides- 
cent polarization,  and  is  best  recognized  by  its  cleavage  and 
characteristic  twinning  lines  as  shown  in  the  figure  on  p.  163. 
Being  soluble  in  carbonated  waters,  it  is  liable  to  complete 
removal,  or  leaves  only  its  impurities  behind  as  a  mark  of  its 
decay. 

Aragonite.  —  Composition :  CaCO3  =  Carbon  dioxide,  44  %  ; 
lime,  56  % . 

This  mineral  has  the  same  chemical  composition  as  calcite, 
but  differs  in  its  crystalline  form  and  specific  gravity.  It 
occurs  with  beds  of  gypsum  and  veins  of  ore,  and  also  in 
stalactitic  and  stalagmitic  forms.  In  small  quantities  it  occurs 
as  a  secondary  product  in  many  trap  rocks  and  basalts,  and  is 
the  substance  of  which  the  shells  of  many  gasteropod  and 
lamellibranch  molluscs  are  composed. 

The  mineral  occurs  nearly  always  in  clustered  aggregates  of 
radiating,  divergent  needles,  and  is  distinguished  from  calcite 
by  its  crystallization  and  cleavage.  As  a  rock  constituent  it  is 
comparatively  unimportant,  but  frequently  occurs  as  a  decom- 
position product  in  basic  eruptives.  This  form  of  calcium 
carbonate,  as  long  ago  pointed  out  by  Sorby,  is  less  stable  than 
calcite,  and  in  many  instances  where  the  substance  has  first 
crystallized  in  the  orthorhombic  form  aragonite,  it  is  found  to 
have  undergone  a  molecular  alteration  into  calcite. 

Dolomite. —  Composition:  (CaMg)CaO3  =  Calcium  carbonate, 
54.35%;  magnesium  carbonate,  45.65%.  Hardness,  3.5-4. 

This  mineral,  like  calcite,  is  a  wide-spread  constituent  of 
rocks,  and  not  infrequently  forms  extensive  masses  which  are 
of  value  as  sources  of  building  material.  It  is  distinguishable 


APATITE   AND   THE    IRON   ORES  27 

from  calcite  by  its  greater  hardness,  higher  specific  gravity, 
and  in  being  but  slightly  acted  on  by  acetic  or  dilute  hydro- 
chloric acid.  In  itself  the  mineral  is  less  susceptible  to  atmos- 
pheric influence  than  calcite,  yielding  much  less  readily  to 
decomposing  agencies  of  a  purely  chemical  nature.  Never- 
theless, Roth1  has  shown  that  in  the  weathering  of  dolomitic 
limestones  the  magnesia  is  sometimes  removed  by  leaching,  in 
greater  proportional  quantities  than  the  more  soluble  lime 
carbonate. 

Apatite.  —  Composition:  Phosphate  of  lime.     Hardness,  5. 

Apatite  is  an  almost  universal  constituent  of  eruptive  rocks, 
both  acid  and  basic,  though  as  a  rule  present  only  in  micro- 
scopic proportions.  In  the  granular  limestones,  schists,  and 
other  metamorphic  and  vein  rocks  it  sometimes  occurs  in  large 
crystals  or  massive  forms  in  such  abundance  as  to  be  of  value 
as  a  source  of  mineral  phosphate  for  fertilizing  purposes.  In 
the  thin  sections  the  apatites  of  eruptive  rocks  are  as  a  rule 
colorless,  and  without  evident  cleavage,  though  presenting 
good  crystallographic  forms.  Rarely  the  mineral  is  pleochroic 
in  red  or  brown  or  bluish  colors.  If  a  drop  of  an  acid  solution 
of  ammonium  molybdate  be  placed  upon  an  apatite  crystal  in 
an  uncovered  slide,  the  mineral  will  be  slowly  dissolved  and 
minute  crystals  of  phosphomolybdate  of  ammonium  be  contem- 
poraneously deposited.  The  process  is  an  easy  one,  readily 
performed  while  the  slide  is  still  011  the  stage,  and  forms  one  of 
the  most  interesting  and  accurate  of  the  many  microchemical 
tests.  Though  present  in  but  small  amounts,  apatite  is  an 
important  constituent,  since  it  is  the  only  common  rock  con- 
stituent containing  the  valuable  element  phosphorus. 

THE  IRON  ORES 

Under  this  head  we  may  conveniently  treat  the  several 
forms  of  iron  oxides  which  commonly  occur  as  rock  constitu- 
ents, and  which  from  their  opacity  in  even  the  thinnest  sec- 
tions, and  similarly  in  crystallographic  outline,  are  separable 
with  difficulty  by  optical  tests  alone. 

Magnetite.  —  Composition:  FeO  -}-  Fe2O3  =  iron  sesquioxide, 
68.97%;  iron  protoxide,  31.03%. 

This  is  a  wide-spread  and  almost  universal  constituent  of 

1  Chemische  u.  Allgemeine  Geologie. 


28  THE   MINERALS   CONSTITUTING   ROCKS 

eruptive  rocks,  occurring  as  a  rule  in  the  form  of  scattering, 
small,  and  rather  inconspicuous  granules,  which  under  the 
microscope  are  characterized  by  a  complete  opacity  and  bluish 
lustre.  When  of  sufficient  size  to  be  distinguished  by  the 
unaided  eye,  magnetite  is  easily  recognized  by  its  brilliant 
lustre,  weight,  and  its  property  of  being  readily  attracted  by 
the  magnet.  It  is  as  a  rule  one  of  the  first  minerals  to  sepa- 
rate out  from  the  molten  magma,  and  hence  presents  good 
crystal  outlines  in  which  octahedral  forms  prevail.  Skeleton 
forms  of  great  beauty  are  not  infrequent.  Magnetite  also 
occurs  as  a  constituent  of  metamorphic  rocks  and  is  some- 
times found  in  large  beds,  constituting  a  valuable  ore  of  iron. 
Under  continual  alternations  of  heat  and  cold,  moisture  and 
dryness,  it  slowly  decomposes,  giving  rise  to  hydrated  sesqui- 
oxides  which  impart  color,  but  no  valuable  qualities,  to  the 
resultant  sands  and  clays. 

Menaccanite  (Ilmenite  or  Titanic  Iron).  —  Composition  : 
(TiFe)2O3,  a  mixture  in  varying  proportions  of  the  oxides 
of  iron  and  titanium. 

This,  like  magnetite,  occurs  in  scattering  granules  as  an 
original  constituent  of  many  eruptive  rocks,  and  also  in  mica- 
ceous lamellar  and  vein-like  masses  in  other  rocks.  Under  the 
microscope  it  shows,  by  incident  light,  a  brownish  rather  than 
bluish  lustre,  but  is  best  recognized  by  its  characteristic  altera- 
tion products,  which  are  whitish,  gummy,  and  opaque.  The 
name  leucoxene  was  given  by  Gumbel  to  the  final  product  of 
this  alteration.  This  form  of  iron  ore  is  extremely  refractory 
to  atmospheric  agencies  and  is  to  be  found  scarcely,  if  any, 
changed  in  the  residuary  materials  resulting  from  the  breaking 
down  of  the  rocks  in  which  it  originated. 

Hematite  (Specular  Iron  Ore.) —  Composition:  Anhydrous  ses- 
quioxide  of  iron,  Fe2O3  =  iron,  70.9  % ;  oxygen,  30.20  %.  H  = 
5.5-6.5. 

This  mineral  occurs  in  varying  proportions  and  under  vary- 
ing conditions  in  rocks  of  all  ages.  In  the  form  of  minute 
scales  of  a  blood-red  color,  it  is  found  not  infrequently  in 
granitic  and  other  eruptive  rocks.  It  occurs,  also,  in  large 
beds,  forming  a  valuable  ore  of  iron.  In  the  amorphous 
condition,  it  may  form  the  cementing  constituent  of  sand- 
stones, and  is  the  cause  of  the  red  color  of  many  rocks,  both 
clastic  and  metamorphic,  and  of  soils  as  well.  The  usual  color- 


LIMONITE    AND   PYRITE  29 

ing  constituent  is,  however,  limonite  or  turgite,  as  noted  below. 
The  specular  and  massive  forms  are  best  recognized  by  opacity, 
brilliant,  black,  metallic  lustre,  and  red  streak. 

Limonite  (Brown  Hematite).  —  Composition:  Hydrous  ses- 
quioxide  of  iron,  H6Fe2O6  +  Fe2O3=iron  sesquioxide,  85.6%; 
water,  14.4  %.  H  =  5-5.5. 

This  is  a  common  constituent  of  rocks  of  all  ages,  but  is  as 
a  rule  wholly  secondary,  resulting  from  the  decomposition  of 
ferruginous  silicates,  sulphides,  and  anhydrous  oxides.  As  a 
coloring  constituent  it  is  even  more  abundant  than  hematite, 
and  like  it  forms  a  valuable  ore  of  iron.  (See  p.  10T.)  Turgite 
(Fe4H2O7)  in  the  form  of  a  brilliant  red  ochreous  material  is 
also  a  common  constituent  of  soils  and  clays  resulting  from  the 
decomposition  of  siliceous  rocks,  and  is  presumably,  like  limo- 
nite, a  product  of  the  spontaneous  hydration  of  the  iron  salts 
thus  set  free.  (See  further  under  Color  of  Soils,  p.  385.) 

Pyrite  (Iron  Pyrites).  —  Composition:  Iron  disulphide,  FeS2 
=  iron,  46.7  % ;  sulphur,  58.3  %.  H  =  6-6.5. 

Two  principal  forms  of  iron  disulphide  occur  in  nature,  alike 
in  chemical  composition,  but  differing  in  forms  of  crystalliza- 
tion and  in  density.  The  one  is  common  pyrites  which  crys- 
tallizes in  the  isometric  system,  and  is  easily  recognized  by  its 
strong  brassy  yellow  color  and  hardness.  Its  usual  form  of 
occurrence  is  that  of  cubes,  the  corners  and  edges  of  which  may 
be  more  or  less  modified  by  secondary  planes,  and  in  concre- 
tionary masses.  The  second  form  marcasite,  also  called  gray, 
white,  or  cockscomb  pyrites,  is  of  lighter  color,  inferior  hard- 
ness and  density,  and  crystallizes  in  the  orthorhombic  system. 
Its  most  common  form  of  occurrence  is  that  of  irregular  con- 
cretionary masses. 

Both  forms  of  pyrite  are  susceptible  to  oxidation  when 
exposed  to  atmospheric  agencies,  though  of  the  two  the  pyrite 
proper  is  much  the  more  refractory. 

Mr.  A.  P.  Brown  has  shown1  that  in  this  form  of  the  com- 
pound a  large  proportion  of  the  iron  exists  in  a  ferric  condition 
while  in  marcasite  it  is  ferrous.  In  other  words,  marcasite  is 
an  unsaturated  compound,  and  hence  unstable.  This  readily 
explains  the  relatively  more  rapid  decomposition  of  the  latter 
mineral.  There  is  also  a  difference  in  the  character  of  the 
products  arising  from  the  decomposition  of  the  two  compounds, 

i  Proc.  American  Philos.  Soc.,  Vol.  XXXIII,  1894,  p.  225. 


30  THE   MINERALS   CONSTITUTING  ROCKS 

pyrite  yielding,  as  a  rule,  limonite  and  free  sulphur,  while  mar- 
casite,  under  the  same  conditions,  yields  ferrous  sulphate,  though 
when  decomposing  under  water,  it  may  also  yield  much  limonite. 
The  sulphate  of  iron,  resulting  from  pyritiferous  decomposition, 
is,  if  present  in  quantity,  injurious  to  plant  growth.  This  fact 
was  well  illustrated  some  years  ago  on  the  west  front  of  the 
National  Museum  at  Washington.  Several  large  masses  of  iron 
sulphide,  too  large  for  exhibition  within  the  building,  were 
placed  here  upon  a  floor  of  cement  bordered  by  a  narrow  strip 
of  lawn.  Under  the  oxidizing  influence  of  rain  and  air  the 
sulphide  became  slowly  converted  into  sulphate  which  was 
washed  down  upon  the  cement  and  thence  into  the  soil,  which 
it  so  poisoned  as  to  kill  the  roots  and  necessitate  an  entire 
resodding. 

The  experiments  of  Prichard, 1  however,  showed  that  the 
presence  of  a  small  amount  of  sulphate  of  iron  in  a  soil  may, 
under  certain  conditions,  be  beneficial,  in  that  it  serves  to  pre- 
vent the  loss  of  ammonia  in  rapidly  decomposing  materials. 
In  processes  involving  slow  decomposition,  its  antiseptic  quali- 
ties render  it  of  doubtful  value. 

Chlorite  (Viridite).  —  Under  the  general  name  chlorite  are 
included  several  minerals  occurring  in  fibres  and  folia,  closely 
resembling  the  micas,  from  which  they  differ  in  their  large  per- 
centage of  water,  and  in  their  folia  being  inelastic.  The  three 
principal  varieties  recognized  are,  ripidolite,  penninite,  and  pro- 
chlorite,  any  one  of  which  may  occur  as  the  essential  constitu- 
ent of  a  chlorite  schist.  Chlorite  as  a  secondary  product  often 
results  from  and  entirely  replaces  the  pyroxene,  hornblende,  or 
mica  in  rocks  of  various  kinds,  and  also  occurs  filling  wholly  or 
in  part  the  amygdaloidal  cavities  of  trap  rocks.  In  this  form 
it  is  frequently  visible  only  with  the  microscope,  and  owing  to 
the  difficulties  in  the  way  of  an  exact  determination  of  its 
mineral  species  is  sometimes  called  viridite.  It  is  this  mineral 
which  gives  the  green  color  to  a  large  share  of  the  more  or 
less  altered  eruptives,  like  the  diabases  and  diorites,  the 
"greenstones"  of  the  older  geologists. 

Serpentine.  —  Composition :  A  hydrous  silicate  of  magnesium 
corresponding  to  the  formula  H4Mg3Si2O9=  silica,  44.1  %  ;  mag- 
nesia, 43.0  %  ;  and  water,  12.9  %. 

The  prevailing  color  is  green,  though  often  spotted  and 
1  Ann.  de  Chernie  et  Physique,  1892. 


GLAUCONITE   AND   THE   ZEOLITES  31 

streaked ;  hence  the  name  from  the  Latin  serpentinus,  a  ser- 
pent. It  has  a  somewhat  greasy  lustre  and  may  be  cut  with  a 
knife,  having  a  hardness  of  about  4  of  the  scale.  The  mineral 
is  always  secondary,  resulting  mainly  from  the  hydration  of 
pure  magnesian  or  lime  magnesian  silicates.  (See  further  on 
p.  115.) 

Glauconite.  —  This  name  is  given  to  a  somewhat  variable 
compound  consisting  essentially  of  silica,  iron,  alumina,  and 
water,  with  smaller  amounts  of  potash,  and  incidentally  lime, 
magnesia,  and  soda.  The  prevailing  color  is  green,  and  as  it 
occurs  in  single  granules  or  granular  aggregates,  it  is  com- 
monly known  as  greensand.  It  is  always  a  secondary  mineral, 
and  has  been  formed  and  is  still  forming  on  many  shallow  sea- 
bottoms  which  receive  fine  sediments  derived  from  the  breaking 
down  of  siliceous  crystalline  rocks.  (See  under  Greensand 
Marl,  p.  133.) 

The  Zeolites.  —  Under  this  head  are  grouped  a  number  of 
minerals  alike  in  being  hydrous  silicates  of  alumina  with  vary- 
ing percentages  of  lime,  potash,  and  soda.  They  are  altogether 
secondary  minerals,  resulting  from  chemical  changes  taking 
place  in  pre-existing  rocks,  and  indicate  not  infrequently  the 
first  or  deep-seated  stages  of  rock  decay.  In  a  more  or  less 
perfect  condition  they  have  been  assumed  to  occur  in  soils, 
having  been  derived  from  the  rocks,  or,  as  is  contended  by  some 
authorities,  having  formed  during  the  process  of  rock  decompo- 
sition or  in  the  soil  itself.  It  is  possible  that  those  constituents 
of  a  soil  which  on  analysis  are  found  to  be  "  soluble  "  as  the 
term  is  ordinarily  used,  may,  in  part  at  least,  have  existed  as 
zeolites.  Hence  their  consideration  in  this  connection  is  of 
importance. 

Out  of  the  22  species  of  minerals  classified  as  zeolites  by 
Dana  in  this  "  System  of  Mineralogy  "  there  are  but  11  which, 
on  account  of  their  abundance  or  chemical  composition,  need 
consideration  here.  The  theoretical  composition  of  these,  as 
indicated  from  a  comparison  of  several  to  many  analyses,  is 
shown  in  the  accompanying  table.  In  addition  to  the  true 
zeolites  are  included  several  other  hydrous  silicates  closely 
related,  both  as  regards  chemical  composition  and  mode  of 
occurrence,  and  which,  in  our  present  discussion,  cannot  well 
be  excluded. 


32 


THE   MINERALS   CONSTITUTING  ROCKS 


SILICA 

(Si02) 

ALUMINA 
(A1203) 

LIME 
(CaO) 

BARIUM 
(BaO) 

POTASH 
(K20) 

SODA 
(Na20) 

WATER 
(H20) 

Ptilolite  .     .     . 
Mordenite    . 
HsulanditG 

70.0 
67.2 

59  2 

11.9 
11.4 

16.8 

4.4 

2.1 
9.2 

.... 

2.4 
3.5 

0.8 
2.3 

10.5 

13.5 

14  8 

Phillipsite    .     . 
Harmotome  . 
Stilbite    .     .     . 
LauinontitG  . 

48.8 
47.1 
57.4 
51  1 

20.7 
16.0 
16.3 
21.7 

7.6 

7.7 
11  9 

20.6 

6.4 
2.1 

1.4 

16.5 
14.1 
47.2 
15  3 

Chabazite     .     . 
Analcite  . 

47.2 
54.5 

20.0 
23.2 

5.5 

.... 

.... 

6.1 
14.1 

21.2 
8  2 

Natrolite 

47.4 

26.8 

16.3 

9.5 

Thomsonite  . 
Prehnite 

36.9 
43  7 

31.4 
24  8 

11.5 

27  1 

.... 

• 

6.4 

13.8 
4  4 

Apophyllite  .     . 

53.7 

25.0 

.... 

5.2 

.... 

16.1 

PLATE   2 


FIG.  1.   Quartz  porphyry  showing  porphyritic  structure. 
FIG.  2.   Quartz  porphyry  showing  flow  structure. 


IV.    THE   PHYSICAL   AND  CHEMICAL   PROPER- 
TIES  OF   ROCKS 

1.     STRUCTURE 

In  considering  the  structure  of  rocks  it  will  facilitate  mat- 
ters to  do  so  under  two  heads :  (1)  the  macroscopic  (or  mega- 
scopic) structures,  or  structures  visible  to  the  unaided  eye 
(macros,  from  Greek  word  fta%/9o?,  signifying  large);  and 

(2)  microscope  structures,  or  those  visible  only  with  the  aid 
of  the  microscope. 

1.  Macroscopic  Structures.  —  From  a  structural  standpoint  all 
rocks  may  be  classified  sufficiently  close  for  present  purposes, 
under  the  heads  of :  (1)  Crystalline,  (2)  vitreous  or  glassy, 

(3)  colloidal,  and   (4)   clastic  or  fragmental.     Of  the  first  of 
these,  ordinary  granite  or  crystalline  marbles  are  good  types, 
being  made  up  wholly  of  crystal  aggregates,  without  interstitial 
amorphous  or  fragmental  material.     The  term  crystalline  gran- 
ular, or  granular  crystalline,  is  often  applied  to  such  as  have  a 
distinctly  granular  structure,  as  do  many  of  the  granitic  rocks. 
Vitreous  or  glassy  structures  are  found  only  among  igneous 
rocks,  and  are  due  always  to  a  cooling  of  the  molten  magma 
too  rapidly  for  the  production  of  crystals.     Obviously,  as  the 
rate  of  cooling  in  rock  masses  must  be  extremely  variable,  so 
we  find  all  intermediate  stages  between  the  completely  glassy 
and  the  crystalline  forms.     To  these  intermediate  stages  such 
names  as  felsitic  and  microlitic  are  given,  names  the  precise 
meaning  of  which  will  be  stated  under  the  head  of  microscopic 
structures.     Rocks  originating  as  chemical  deposits,  and  which 
have  since  undergone  no  structural  changes,  often  present  a 
jelly  or  glue  like  structure  known  as  colloidal.     Such  are  exem- 
plified in  the  flints  from  the  English  chalk  cliffs,  the  siliceous 
sinters  from  the  Yellowstone  National  Park,  and  by  various 
other  forms  of  silica,  as  opal,  agate,  etc.,  and  occasionally  by 
serpentines. 

n  33 


31          PHYSICAL  AND   CHEMICAL   PROPERTIES   OF  ROCKS 

A  clastic  or  fragmental  structure  is  found  only  in  secondary 
rocks,  and  is  the  result  of  a  breaking  down  or  disintegration  of 
pre-existing  rocks,  and  a  reconsolidation  of  their  particles  with- 
out crystallization.  There  are  many  minor  points  of  structure, 
some  of  which  are  common  to  all  of  the  primary  groups  above 
mentioned,  while  others  are  limited  to  one  or  more.  Rocks 
which  are  made  up  of  distinct  grains,  whether  crystalline  or 
fragmental,  are  spoken  of  as  granular  ;  when  the  structure  be- 
comes too  tine  and  dense  for  macroscopic  determination  it  is 
spoken  of  as  compact,  though  there  is  no  reason  why  the  term 
should  not  equally  well  be  applied  to  the  coarser  grained  rocks 
in  which  the  individual  grains  are  closely  cohering  without 
interstices.  The  term  massive  is  applied  to  such  igneous  rocks 
as  show  no  signs  of  bedding  or  stratification,  while  limestones, 
sandstones,  and  such  other  rocks  as  are  arranged  in  more  or 
less  parallel  layers  are  described  as  stratified.  (See  Fig.  1, 
PI.  13.)  The  name  foliated  or  schistose  is  given  to  a  rock  in 
which  the  arrangement  of  the  constituent  minerals  in  parallel 
planes  is  sufficiently  marked  to  cause  it  to  split  in  this  direction 
more  readily  than  in  any  other.  Not  infrequently  the  quartzes 
or  feldspars  occur  in  lens-shaped  forms  about  which  curve  the 
hornblende  or  mica  folia  as  shown  in  Fig.  2,  PI.  13.  As  ex- 
plained elsewhere,  this  structure  may  be  due  to  original  deposi- 
tion or  may  be  secondary.  In  eruptive  rocks  a  fluidal  or  fluxion 
structure  is  not  uncommon,  as  shown  in  Fig.  2,  PI.  2,  and  is  due 
to  the  onward  flowing  of  the  mass  while  gradually  cooling  and 
passing  into  a  solid  state.  Eruptive  magmas  at  the  time  of 
their  extrusion  contain  more  or  less  moisture,  which,  being 
highly  heated,  expands  whenever  sufficient  force  is  developed 
to  overcome  the  pressure  of  the  overlying  mass.  In  this  way 
are  formed  innumerable  cavities  or  bubbles,  comparable  to  the 
cavities  caused  by  carbonic  acid  from  the  yeast  in  well-raised 
bread.  Such  cavities  are  called  vesicles,  and  the  rocks  contain- 
ing them  are  vesicular  (Fig.  2,  PI.  3).  By  the  subsequent 
action  of  percolating  waters  these  cavities  may  become  filled 
with  a  variety  of  secondary  minerals,  among  which  chalcedony, 
epidote,  calcite,  and  various  zeolites  are  not  uncommon.  Such 
refilled  cavities  are  called  amygdules,  from  the  Greek  word 
a^v^a\ov,  an  almond,  in  allusion  to  their  shape,  and  the  rocks 
containing  them  are  therefore  described  as  amygdaloidal.  The 
upper  part  of  a  lava  flow  not  infrequently  cools  in  peculiar  ropy 


PLATE   3 


FIG.  1.  Basalt  showing  slaggy  structure.  FIG.  2.  Basalt  showing  vesicular  structure. 


MACROSCOPIC   STRUCTURE  35 

forms  like  the  slag  from  a  smelting  furnace.  Such  forms  are 
known  as  slaggy.  (See  Fig.  1,  PL  3.) 

When  a  rock  consists  of  a  compact,  glassy,  or  fine  and  evenly 
crystalline  ground-mass,  throughout  which  are  scattered  larger 
crystals,  usually  of  feldspar,  the  structure  is  said  to  be  porphy- 
ritic  (Fig.  1,  PI.  2).  This  structure  is  quite  common  in  granite, 
but  is  not  particularly  noticeable,  owing  to  the  slight  contrast  in 
color  between  the  larger  crystals  and  the  finer  ground-mass.  It 
is  most  noticeable  in  such  effusive  eruptives  as  the  quartz  por- 
phyries, in  which,  as  is  the  case  with  some  of  those  of  eastern 
Massachusetts,  the  ground-mass  is  exceedingly  dense  and  com- 
pact and  of  a  black  or  red  color,  while  the  large  feldspar 
crystals  are  white  and  stand  out  in  very  marked  contrasts. 
This  structure  is  so  striking  in  appearance  that  rocks  possess- 
ing it  in  any  marked  degree  are  popularly  called  porphyries, 
whatever  may  be  their  mineral  composition.  The  term  por- 
phyry is  said  to  have  been  originally  applied  to  certain  kinds 
of  igneous  rocks  of  a  reddish  or  purple  color,  such  as  the 
celebrated  red  porphyry  or  "  roseo  antico  "  of  Egypt.  The 
word  is  now  used  by  the  best  authorities  almost  wholly  in  its 
adjective  sense,  since  any  rock  may  possess  this  structure 
whatever  its  origin  or  composition  may  be. 

Glassy  rocks  on  cooling  sometimes  have  developed  in  them 
a  series  of  concentric  cracks  whereby  the  rock  on  a  broken  sur- 
face shows  numerous  rounded  or  globular  bodies  with  an  onion- 
like  shell.  This  structure,  which  may  be  visible  only  with  a 
microscope,  is  known  as  perlitic.  It  is  not  uncommon  in  glassy 
forms  of  Hungarian  trachytes. 

Glassy  and  felsitic  eruptives,  particularly  of  the  liparite  and 
quartz  porphyry  groups,  frequently  show  spherulitic  masses  of 
all  sizes,  from  microscopic  to  several  inches  or  even  feet  in 
diameter,  usually  with  a  well-defined  radiating  structure  and 
which  are  due  to  incipient  crystallization.  Such  are  known  as 
spherulites,  and  hence  rocks  in  which  they  occur  are  described 
as  spherulitic.1 

A  concretionary  structure  is  not  infrequently  developed  in 
rocks  either  as  a  primary  structure  or  as  due  to  segregating 
processes  acting  subsequent  to  the  formation  of  the  rocks  in 

1  The  structure  and  origin  of  these  forms  has  been  worked  out  in  detail  by 
Whitman  Cross.  Bull.  Philosophical  Society  of  Washington,  Vol.  XI,  1891, 
pp.  411-462. 


36          PHYSICAL   AND   CHEMICAL  PROPERTIES   OF  ROCKS 

which  they  are  found.  Many  of  the  forms  thus  developed  are 
peculiarly  deceptive,  and  it  may  not  be  out  of  place  to  enter 
into  a  discussion  of  their  nature  and  origin  with  some  detail. 

On  genetic  grounds  we  may  divide  such  forms,  as  intimated 
above,  into  two  groups:  (.A)  Primary  concretions,  formed  con- 
temporaneously with  the  rocks  in  which  they  are  found,  and 
(^)  secondary  concretions,  or  those  which  are  due  to  segregat- 
ing influences  acting  subsequent  to  the  formation  of  the  rocks 
of  which  they  now  form  a  part.  All  are  due  to  that  peculiar 
and  little  understood  tendency  which  atoms  or  molecules  of 
like  nature  so  often  manifest  in  concreting  or  gathering  in 
amorphous  masses  or  concentric  layers  about  some  foreign  body 
which  serves  as  a  primary  point  of  attachment.  The  extreme 
development  of  this  tendency  is  seen  in  crystallization,  of  which 
we  may  perhaps  regard  this  first  form  of  concretionary  structure 
as  incipient  stages.  Under  primary  concretions  may  be  included 
the  flint  and  chalcedonic  nodules  found  in  chalk  and  the  older 
limestones,  the  material  of  which  was  in  part  without  doubt 
derived  from  the  siliceous  remains  of  diatoms  and  sponges. 
Such  sometimes  occur  in  the  form  of  lenticular  nodules  with 
or  without  an  appreciable  concentric  structure  and  lying  in 
parallel  layers  or  beds,  sometimes  continuous  for  long  distances. 
Clay  iron  stone,  an  impure  carbonate  of  iron,  occurs  character- 
istically in  this  form.  These  latter  often  crack  on  drying 
and  consequent  shrinkage,  the  cracks  extending  from  within 
outward.  In  these  cracks  calcite  is  subsequently  deposited, 
whereby  the  nodule  is  divided  up  into  septa  of  a  white  or 
yellowish  color.  On  being  cut  and  polished,  these  often  form 
beautiful  and  unique  objects.  To  such  the  name  septarian 
nodule  is  commonly  given.  (See  Fig.  2,  PI.  9.)  The  car- 
bonate of  lime  in  inland  lakes  and  seas  may  not  infrequently 
become  deposited  in  the  form  of  thin  pellicles  about  a  minute, 
perhaps  microscopic  nucleus,  forming  small,  spherical  bodies 
which,  when  ultimately  consolidated  into  beds,  give  rise  to  the 
oolitic  and  pisolitic  limestones.  (See  p.  143.)  All  primary 
concretions  are  not,  however,  chemical  deposits  ;  but,  rather, 
aggregates  of  mineral  particles  in  a  finely  fragmental  condition. 

Such  are  the  clay  concretions  which  are  found  in  the  beds 
of  streams  and  lakes,  and  which  may  not  so  closely  simu- 
late animal  forms  as  to  be  very  misleading.  The  manner  in 
which  concretions  of  this  nature  are  formed  was  shown  in  a 


MACROSCOPIC   STRUCTURE  37 

very  interesting  manner  a  few  years  ago  during  the  process  of 
the  work  of  filling  in  the  so-called  Potomac  flats,  on  the  river 
front  at  Washington,  District  of  Columbia.  For  the  double 
purpose  of  raising  the  flats  and  deepening  the  channel,  gigantic 
pumps  were  employed  which  raised  the  sediment  from  the  river 
bottom  in  the  form  of  a  very  thin  mud  and  forced  it  through 
iron  pipes  to  the  flats,  where  it  flowed  out,  spreading  quietly 
over  the  surface.  The  material  of  this  mud  was  mainly  fine 
siliceous  sand  and  clay  intermingled  with  occasional  fresh- 
water shells  and  plant  debris.  As  this  mud  flowed  quietly 
from  the  mouth  of  the  pipe  and  spread  out  over  the  surface, 
the  clayey  particles  began  immediately  to  separate  from  the 
siliceous  sand  in  the  form  of  concretionary  balls,  and  in  the 
course  of  a  very  short  time  these  would  grow  to  be  several 
inches  in  diameter.  Such,  owing  to  the  rapidity  of  their 
formation,  contained  a  large  amount  of  sand  and  shells,  though 
clayey  matter  predominated. 

In  crystalline  rocks  concretionary  structure  is  rarely  devel- 
oped. Cases  such  as  shown  on  Plate  8  are  quite  unique,  and 
in  the  case  of  the  orbicular  diorite  of  the  greatest  interest  on 
account  of  the  beauty  of  the  stone  and  its  adaptability  for 
small  ornamentation. 

Concretionary  structure  of  a  secondary  nature  may  be  de- 
veloped through  the  process  of  weathering.  Thus,  by  the 
oxidizing  action  of  meteoric  waters  percolating  through  a 
porous  sand  or  sandstone,  included  nodules  of  iron  disulphide 
(pyrite)  may  be  converted  into  an  oxide  which  gradually 
segregates  in  zones  about  the  original  nodule.  This  oxide, 
by  its  cementing  action,  binds  the  grains  together  in  the  form 
of  a  hard  crust,  leaving  the  central  portion,  formerly  filled  by 
pyrite,  either  empty  or  occupied  by  loose  sand.1  A  zonal 
banding  or  shelly  structure  closely  simulating  concretionary 
structure  is  common  in  rocks  more  or  less  weathered  and 
decomposed,  but  which  is  due  not  to  original  deposition  or 
crystallization  of  mineral  matter  about  a  centre,  but  rather  to 
the  weathering  of  jointed  blocks,  the  various  chemical  agencies 
acting  from  without  inward. 

A  botryoidal  structure  is  not  infrequent  among  rocks  and 
minerals  of  chemical  origin.  It  is,  as  a  rule,  confined  to  such 

1  See  On  the  Formation  of  Sandstone  Concretions,  Proceedings  U.  S.  National 
Museum,  Vol.  XVII,  pp.  87,  88. 


38          PHYSICAL  AND   CHEMICAL  PROPERTIES   OF   ROCKS 

as  are  amorphous  or  radiating  crystalline  aggregates  of  a  single 
mineral,  such  as  chalcedony  or  the  hematite  iron  ores.  (See 
Fig.  1,  PL  9.) 

A  brecciated  structure,  produced  by  the  presence  of  angular 
fragments  in  a  finer  ground,  is  of  common  occurrence  among 
fragmental  rocks  (the  breccias),  but  is  more  rare  among  the 
crystallines.  It  is  sometimes  produced  in  volcanic  rocks  by  the 
imbedding  in  the  still  pasty  magma  of  angular  fragments  of 
previously  consolidated  material,  as  shown  in  Fig.  2,  PI.  4. 
Columnar  structure,  though  comparatively  common  as  the 
structure  of  a  geological  body,  is  rarely  developed  among  the 
constituents  of  the  rock  itself.  The  columnar  structure  of 
many  lavas  and  dike  rocks  has  already  been  alluded  to  :  oc- 
casionally the  mineral  constituents  of  some  secondary  rocks 
are  arranged  after  this  manner.  A  cavernous  or  cellular  struct- 
ure is  not  infrequently  developed  through  the  removal  by 
solution  of  some  constituent  or  the  weathering  out  of  a  fossil. 
As  an  original  structure  it  occurs  in  many  rocks  of  chemical 
origin  as  the  stalagmitic  deposits  in  caves,  travertines,  etc. 

A  laminated  or  banded  structure,  due  to  the  arrangement  of 
the  constituents  in  parallel  layers  or  bands,  is  common  in  rocks 
of  sedimentary  origin,  particularly  in  sandstones  and  shales. 

2.  Microscopic  Structures.  —  Many,  if  not  indeed  the  majority, 
of  rocks  are  so  fine  grained  and  compact  that  little  of  their 
mineral  nature  or  structural  features  can  be  learned  from  exami- 
nation by  the  unaided  eye.  This  difficulty  made  itself  apparent 
very  early  in  the  history  of  geological  science,  and  to  it  is  per- 
haps due,  more  than  to  any  other  single  cause,  the  apparent 
crudities  and  fallacies  of  the  early  workers.  As  long  ago  as 
1663,  the  microscope  had  been  to  some  extent  utilized  for  the 
examination  of  minerals ;  but  its  application  to  the  study  of 
rocks  remained  long  unrecognized,  though  early  in  the  present 
century  Cordier  and  others  utilized  it  in  the  study  of  rocks  in 
a  pulverized  condition.  It  was  not  until  about  1850,  when  the 
subject  was  taken  up  by  H.  Clifton  Sorby  of  England,  that  the 
possibility  of  studying  rocks  in  thin  sections  under  the  micro- 
scope began  to  be  appreciated.  Even  then  the  idea  failed  to 
bear  its  legitimate  fruits  until  transplanted  to  German  soils, 
where,  under  the  fostering  care  of  Professor  Zirkel  of  Leipzig, 
it  soon  began  to  yield  an  abundant  harvest ;  and  to-day  the 
branch  of  the  science  of  geology  known  as  microscopical  pe- 


PLATE   4 


MICROSCOPIC   STRUCTURE  39 

trography  holds  a  prominent  place  in  all  the  leading  universi- 
ties, both  domestic  and  foreign.  The  efficiency  of  the  method 
is  based  upon  the  fact  that  every  crystallized  mineral  has  cer- 
tain definite  optical  properties  ;  i.e.  when  cut  in  such  a  way  as 
to  allow  the  light  to  pass  through  it,  will  act  upon  this  light  in 
a  manner  sufficiently  characteristic  to  enable  one  working  with 
an  instrument  combining  the  properties  of  a  microscope  and 
stauroscope  to  ascertain  at  least  to  what  crystalline,  system  it 
belongs,  and  in  most  cases  by  studying  also  the  crystal  outlines 
and  lines  of  cleavage  the  mineral  species  as  well.  To  enter 
upon  a  detailed  description  of  the  method  by  which  this  is  done 
would  be  out  of  place  here,  since  it  involves  the  polarization  of 
light  and  other  subjects  which  must  be  studied  elsewhere.  The 
reader  is  referred  to  any  authoritative  work  on  the  subject  of 
light,  and  to  Professor  J.  P.  Idding's  translation  of  Professor 
Rosenbusch's  work  on  optical  mineralogy.1 

This  method  of  study  is  of  value,  not  merely  as  an  aid  in 
determining  the  mineralogical  composition  of  a  rock,  but  also, 
and  what  is  often  of  more  importance,  its  structure  and  the 
various  changes  which  have  taken  place  in  it  since  its  first 
consolidation.  Rocks  are  not  the  definite  and  unchangeable 
mineral  compounds  they  were  once  considered,  but  are  rather 
ever- varying  aggregates  of  minerals,  which,  even  in  themselves 
undergo  structural  and  chemical  changes  almost  without  num- 
ber. It  is  a  common  matter  to  find  rock  masses  which  may 
have  had  originally  the  mineral  composition  and  structure  of 
diabase,  but  which  now  are  mere  aggregates  of  secondary  prod- 
ucts, such  as  chlorite,  epidote,  iron  oxides,  and  kaolin,  with 
perhaps  scarcely  a  trace  of  the  unaltered  original  constituents  ; 
yet  the  rock  mass  retains  its  geological  identity,  and  to  the 
naked  eye  shows  little,  if  any,  sign  of  the  changes  that  have 
gone  on.  These  and  other  changes  are  in  part  chemical  and  in 
part  structural  or  molecular.  A  very  common  mineral  trans- 
formation in  basic  rocks  is  that  from  augite  to  hornblende. 
This  takes  place  merely  through  a  molecular  readjustment  of 
the  particles,  whereby  the  augite,  with  its  gray  or  brown  colors 
and  rectangular  cleavages,  passes  by  uralitic  stages  over  into  a 
green  hornblende,  a  mineral  of  the  same  chemical  composition, 
but  of  different  crystallographic  form.  This  transformation  in 

1  Microscopic  Physiography  of  Rock-making  Minerals,  Wiley  &  Son,  New 
York.  See  also  Professor  A.  Barkers'  Petrology  for  Students. 


40          PHYSICAL   AND   CHEMICAL  PROPERTIES   OF  ROCKS 

its  incompleted  state  is  shown  in  the  accompanying  figure,  in 
which  the  central,  nearly  colorless  portion  with  rectangular 
cleavage  represents  the  original  augite,  while  the  outer  dotted 

portion  with  cleavage  lines  cutting  at 
sharp  and  obtuse  angles  is  the  second- 
ary hornblende.  This  change  is  due 
to  slow  and  gradual  pressure  exerted 
through  unknown  periods  of  time  upon 
the  rock  masses,  and  the  final  result  is 
the  production  of  a  rock  of  entirely 
different  type  and  structure  from  that 
which  originally  cooled  from  the  molt- 
en magma.  The  change  such  as  above 
FIG.  i.— Augite  partially  described  is  further  alluded  to  in  the 

chapter  on  metamorphism. 

This  science  of  microscopic  petrography,  as  it  is  technically 
called,  has  also  been  productive  of  equally  important  results  in 
other  lines.  As  an  instance  of  this  may  be  mentioned  the  dis- 
covery that  the  structural  features  of  a  rock  are  dependent,  not 
upon  its  chemical  composition  or  geological  age,  but  upon  the 
conditions  under  which  it  cooled  from  a  molten  magma,  portions 
of  the  same  rock  varying  all  the  way  from  holocrystalline 
granular  through  porphyritic  to  glassy  forms.  To  this  fact 
allusion  has  already  been  made. 

The  general  subject  of  the  microscopic  structure  of  rocks  of 
various  kinds,  will  be  discussed  more  fully  in  describing  the 
rocks  themselves.  Nevertheless,  as  in  describing  these  struct- 
ures it  has  become  necessary  to  use  sundry  technical  terms,  it 
will  be  well  to  refer  to  them  briefly  here. 

When  a  rock  is  made  up  wholly  of  crystalline  matter,  it  is 
spoken  of  as  holocrystalline  ;  when,  however,  it  shows  interstitial 
glassy  or  felsitic  matter,  it  is  hypocrystalline.  Rocks  wholly 
without  crystalline  secretions  are  amorphous.  The  glassy,  or 
felsitic  matter  occupying  the  interstices  of  the  other  constitu- 
ents is  spoken  of  as  the  base.  This  base,  together  with  the 
microlites  and  smaller  crystallizations  of  the  second  generation, 
is  called  the  ground-mass;  such  may  be  made  up  of  microlites  — 
small  needle-like  crystals  imperfectly  developed  —  when  it  is 
called  microlitic,  or  of  a  dense  aggregate  of  quartzose,  felds- 
pathic  and  other  materials,  when  it  is  known  as  felsitic.  The 
larger  crystals  developed  in  a  glassy,  felsitic,  microlitic,  or  finely 


PLATE   5 


FIG.  1.   Microstructure  of  granite. 

FIG.  2.   Microstructure  of  micropegmatite. 

FIG.  3.   Microstructure  of  quartz  porphyry. 


FIG.  4.  Microstructure  of  porphyritic  obsidian. 
FIG.  5.   Microstructure  of  trachyte. 
FIG.  6.   Microstructure  of  serpentine. 


MICROSCOPIC   STRUCTURE  41 

granular  micro  crystalline  ground-mass  are  called  phenocrysts. 
When  a  mineral  in  a  rock  shows  good  crystal  outlines,  having 
been  uninfluenced  in  its  growth  by  the  proximity  of  other 
minerals,  it  is  called  idiomorphic :  when,  however,  its  outline  is 
due  not  to  cryst/allographic  forces,  but  to  interference  —  to  the 
action  of  external  forces  —  it  is  allotriomorphic.  Many  rocks 
show  indications  of  two  or  more  periods  of  crystallization, 
whereby  minerals  of  the  same  species  may  be  developed.  Thus 
in  a  molten  magma  the  augites  may  begin  to  form  under  such 
conditions  that  for  some  time  their  growth  is  unimpeded  and 
they  take  on  large  and  well-developed  forms.  After  a  time, 
owing  to  changed  conditions,  their  growth  is  stopped,  and  the 
rock  solidifies  with  a  new  crop  of  smaller  and  less  perfectly 
developed  forms.  It  is  customary  to  speak  of  such  a  mineral 
as  occurring  in  crystals  of  two  generations.  In  the  case  above 
described,  the  first  developed  form  the  porphyritic  constitu- 
ents, the  phenocrysts,  while  the  latter  formed  are  a  part  of 
the  ground-mass.  Vitreous  or  glassy  rocks  not  infrequently 
show,  under  the  microscope,  minute,  hair-like  or  rod-shaped 
forms,  representing  the  first  stages  of  crystallization,  but  in 
which  the  process  was  arrested  before  they  were  sufficiently 
developed  to  render  possible  an  accurate  determination  of 
their  mineral  nature.  Such  are  termed  crystallites;  those  in 
drop-shaped  or  globular  forms  being  called  globulites,  the 
rod-shaped  ones  belonites,  and  the  twisted,  hair-like  forms 
trie  kite s. 

Tile  wide  variation  in  microstructure  in  rocks  of  essentially 
the  same  chemical  composition,  but  which  have  cooled  under 
the  varying  conditions  indicated  above,  is  shown  in  Figs.  1  to 
4  of  PI.  5,  Fig.  1  being  a  holocrystalline  type,  and  Fig.  4  one 
almost  completely  glassy,  the  first  being  a  deep-seated  rock,  and 
the  last  a  surface  lava  flow.  Intermediate  structures  are  often 
produced  through  a  beginning  of  crystallization  at  certain 
depths  below  the  surface,  after  which,  and  while  a  portion  of 
the  magma  was  still  fluid,  it  was  pushed  upward  toward  the 
surface,  or  brought  under  such  other  conditions  as  resulted  in 
a  more  rapid  cooling,  the  final  result  being  a  glassy,  or  micro- 
crystalline  rock  with  scattering  porphyritic  crystals,  or  pheno- 
crysts. It  has  not  infrequently  happened  that,  subsequent  to 
the  formation  of  these  earliest  products  of  crystallization,  a 
second  elevation  of  temperatures  has  taken  place  whereby  the 


42          PHYSICAL   AND   CHEMICAL  PROPERTIES   OF  ROCKS 

magma  has  eaten  into  or  corroded  them,  as  is  the  case  with 
the  quartz  crystal  shown  in  the  centre  of  Fig.  3  of  PI.  5. 

Inasmuch  as  this  study  by  the  microscope  involves  the  prepa- 
ration of  thin  sections,  a  brief  description  of  the  methods  pur- 
sued may  well  be  given  here.  The  fact  that  a  chip  of  rock, 
however  dense,  can,  without  breaking,  be  ground  so  thin  as 
to  be  transparent,  may  at  first  seem  strange,  but  in  reality  it 
is  readily  accomplished.  The  work  requires  only  patience  and 
the  skill  which  comes  from  practice.  A  small  chip  of  the  rock, 
about  the  size  of  a  nickel  five-cent  piece,  is  broken  off  with  a 
hammer,  care  being  taken  to  get  it  as  thin  as  possible  without 
fracturing.  One  side  of  this  is  then  ground  flat  and  smooth  by 
rubbing  it  in  water  and  emery  on  a  smooth,  cast-iron  plate. 
Toward  the  close  of  the  process  fine  flour  of  emery  should  be 
used,  as  the  final  surface  must  be  very  smooth  and  free  from 
scratches.  This  chip  is  then  cemented  smooth  side  down  on 
a  piece  of  ordinary  double-thick  window  glass,  a  convenient 
size  being  about  2x1  inches,  the  cementing  material  being 
Canada  balsam  which  has  been  evaporated  to  the  extent  that, 
when  cold,  it  is  sufficiently  hard  to  hold  firmly,  is  not  at  all 
sticky,  but  yet  is  not  so  hard  as  to  be  brittle.  The  exact  degree 
can  only  be  learned  by  experience ;  a  hardness  such  as  to  be 
barely  indented  by  the  thumb  nail  will  be  found  about  right. 
This  operation  of  cementing  will  be  best  done  by  means  of  a 
thin  iron  plate  laid  horizontally  on  a  support  and  heated  not  too 
hot  by  a  lamp  beneath.  The  glass  with  the  balsam  upon  it  is 
heated  to  the  right  temperature,  the  balsam  being  fluid  and  free 
from  bubbles.  The  rock  chip,  heated  sufficiently  to  expel  all 
moisture,  is  then  pressed  firmly  into  the  balsam,  in  such  a  way 
as  to  exclude  air  bubbles,  and  brought  within  as  close  contact 
with  the  glass  as  possible.  It  is  then  removed  from  the  iron, 
plate  and  allowed  to  cool,  when  the  grinding  process  is  resumed, 
the  glass  plate  serving  merely  as  support  for  the  film  of  stone 
and  something  for  the  fingers  to  hold  by.  Being  transparent, 
the  worker  can  see  just  how  the  grinding  is  progressing  without 
continually  stopping  to  examine.  When  sufficiently  thin,  — 
usually  from  -^-^  to  g^-  of  an  inch,  —  the  film  is  remounted  as 
follows  :  While  on  the  thick  glass  on  which  it  was  ground,  it 
is  thoroughly  washed  with  a  brush  —  an  ordinary  tooth-brush 
serves  well  —  to  get  rid  of  all  particles  of  emery  and  other  dirt 
that  may  adhere.  It  is  then  washed  in  alcohol  to  get  rid  of  the 


THE   SPECIFIC   GRAVITY   OF    ROCKS 


43 


old  hard  balsam,  which  is  usually  quite  dirty  from  mud  pro- 
duced in  grinding.  Fresh  mounting  slips  and  clean  cover 
glasses  being  ready,  the  first  is  laid  upon  the  warm  iron  plate 
with  a  couple  of  drops  of  fresh  balsam  in  the  centre,  and  allowed 
to  heat  until  it  just  begins  to  smoke.  Care  must  here  be  exer- 
cised, as,  if  heated  too  much,  the  balsam  becomes  hard  and 
brittle,  and  if  too  little,  the  mount  is  sticky  from  the  balsam 
which  constantly  oozes  from  under  the  cover.  The  thick  glass, 
with  its  film  of  stone  still  adhering,  is  likewise  laid  upon  the 
warm  iron  plate,  and  a  drop  of  fresh  balsam  placed  upon  the  film. 
This  is  then  gently  heated,  and  the  cover-glass,  first  warmed, 
gently  laid  upon  it  —  one  edge  placed  in  position  and  loAvered 
gradually  in  such  a  manner  as  to  force  out  any  accidental  air 
bubbles,  being  finally  pressed  flat  down  against  the  stone  film. 
The  film  itself,  if  sufficiently  warmed,  no  longer  adheres  to  the 
thick  glass,  and  may  be  removed  to  the  ^ 
clean  slip  for  its  final  mounting.  This  is 
best  accomplished  by  taking  up  the  thick 
glass  by  means  of  a  pair  of  forceps  and 
pushing  cover-glass  and  film  together,  with 
a  needle  point  set  in  a  handle,  off  into  the 
balsam  on  the  new  slide.  The  cover-glass 
here  serves  merely  as  a  support  for  the  thin 
film  during  the  process  of  transferring. 
Without  it  there  is  danger  of  breakage. 
When  fairly  transferred,  the  new  slide  is 
removed  from  the  hot  plate,  the  cover 
pressed  close  down  against  the  film,  ad- 
justed in  proper  position  and  allowed  to  cool.  FlG- 2-  —  Mounted  thin 

rrn  nil  ^  ii  Section  of  rock. 

Ine  superfluous    balsam   may  be  then   re- 
moved with  a  hot  knife  and  the  section  finally  washed  in  alcohol. 
Thus  completed,  it  forms  the  "  thin  section  "  of  the  petrologist. 


2.     THE  SPECIFIC  GRAVITY  OF  BOCKS 

The  term  specific  gravity  is  used  to  designate  the  weight  of 
any  substance  when  compared  with  an  equal  volume  of  distilled 
water  at  a  temperature  of  4°  C.  This  property  is  therefore 
dependent  upon  the  specific  gravity  of  its  various  constituents 
and  their  relative  proportions.  The  exact  or  true  specific 
gravity  of  a  rock  may  be  obscured  by  its  structure.  Thus  an 


44          PHYSICAL   AND   CHEMICAL   PROPERTIES   OF   ROCKS 

obsidian  pumice  will  float  upon  water,  buoyed  up  by  the  air 
contained  in  its  innumerable  vesicles,  while  a  compact  obsidian 
of  precisely  the  same  chemical  composition  will  sink  almost 
instantly.  This  property  of  any  subject  is  spoken  of  as  its 
apparent  specific  gravity  in  distinction  from  the  actual  com- 
parative weight,  bulk  for  bulk,  of  its  constituent  parts,  which 
could  in  the  case  of  a  pumice  be  obtained  only  by  finely  pul- 
verizing so  as  to  admit  the  water  into  all  its  pores.  Inasmuch 
as  the  structural  peculiarities  of  any  igneous  rock — as  will  be 
noted  later  —  are  dependent  upon  the  condition  under  which  it 
cooled,  it  is  instructive  to  notice  that  a  crystalline  aggregate 
has  a  higher  specific  gravity,  i.e.  a  greater  weight,  bulk  for 
bulk,  than  does  a  glassy,  non-crystalline  rock  of  the  same  chemi- 
cal composition.  The  property  is  therefore  dependent  upon 
chemical  (and  consequently  mineral)  composition  and  struct- 
ure, and  as  a  very  general  rule  it  may  be  said  that  among  the 
siliceous  rocks  those  which  contain  the  largest  amount  of  silica 
are  the  lightest,  while  those  with  a  comparatively  small  amount, 
but  which  are  correspondingly  rich  in  iron,  lime,  and  magnesian 
constituents,  are  proportionately  heavy. 

3.     THE  CHEMICAL  COMPOSITION  OF  ROCKS 

This  varies  naturally  with  their  mineral  composition.  It  is 
customary  to  speak  of  sedimentary  rocks  as  calcareous,  sili- 
ceous, ferruginous,  or  argillaceous,  accordingly  as  lime,  silica, 
iron  oxides,  or  clayey  matter  are  prominent  constituents. 
Among  eruptive  rocks  it  is  customary  to  speak  of  those  show- 
ing, on  analysis,  upwards  of  60  %  silica  as  acidic,  and  those 
showing  less  than  50  %,  but  rich  in  iron,  lime,  and  magnesian 
constituents,  as  basic.  The  extremes,  as  will  be  noted,  are  rep- 
resented by  the  rocks  of  the  granite  and  peridotite  groups. 

A  series  illustrating  the  above-mentioned  properties  may  be 
arranged  as  below.  With  the  eruptive  rocks  only  the  silica 
percentages  are  here  given.  •  The  results  of  the  complete  chemi- 
cal analysis  of  each  variety  are  given  further  on,  in  the  pages 
devoted  to  their  description. 


THE  CHEMICAL  COMPOSITION  OF  ROCKS 


45 


(1)    STRATIFIED   ROCKS 


KIND 

SPECIFIC  GRAVITY 

COMPOSITION 

Calcareous  : 
Compact  limestone  .    .     . 
Crystalline  limestone   .     . 
Compact  dolomite    .     .     . 
Crystalline  dolomite     .     . 
Siliceous  : 
Gneiss 

1  2.6  to  2.8 
1  2.8  to  2.95 

2  6  to  2  7 

Carbonate  of  lime. 
Carbonate  of  lime  and  magnesia. 

Same  as  granite 

Siliceous  sandstone  .    .    . 
Schist 

2.6 
2  6  to  2  8 

Mainly  silica. 
60  to  80  per  cent  silica 

Argillaceous  : 
Clay  slate  (argillite)     .    . 

2.5 

Mainly  silicate  of  aluminum. 

(2)    ERUPTIVE   ROCKS 


KIND 

SPECIFIC  GRAVITY 

PER  CENT  SILICA 

Acidic  group  : 
Granite 

2  58  to  2  73 

77  65  to  62  90 

Liparite 

2  53  to  2  70 

76  06  to  67  61 

Obsidian 

2.26  to  2  41 

82  80  to  71  19 

Obsidian  pumice 

Floats  on  water. 

82.80  to  71.19 

Intermediate  group  : 
Syenite     

2.73  to  2.86 

72.20  to  54.65 

Trachyte  

2.70  to  2.80 

64.00  to  60.00 

Hyalotrachyte  
Andesite 

2.40  to  2.50 
2  54  to  2  79 

64.00  to  60.00 
66.75  to  54.73 

Basic  group  : 
Diabase 

2  66  to  2.88 

50.00  to  48.00 

Basalt  .    .         

2.90  to  3.10 

50.59  to  40.74 

Peridotite     

3.22  to  3.29 

42.65  to  33.73 

Peridotite  (iron  rich)     
Peridotite  (meteorite) 

3.86 
3  51 

23.00 
37.70 

4.     THE  COLOR  OF  ROCKS 


The  color  of  a  rock  is  dependent  upon  a  variety  of  circum- 
stances, but  which  may  all  be  generalized  under  the  heads  of 
mineral  and  chemical  composition  and  physical  condition.  Iron 
and  carbon,  in  some  of  their  forms,  are  the  common  coloring 


46          PHYSICAL  AND   CHEMICAL   PROPERTIES   OF   ROCKS 

substances  and  the  only  ones  that  need  be  considered  here. 
The  yellow,  brown,  and  red  colors,  common  to  fragmental  rocks, 
are  due  almost  wholly  to  free  oxides  of  iron.  The  gray,  green, 
dull  brown,  and  even  black  colors  of  crystalline  rocks  are  due 
to  the  presence  of  free  iron  oxides  or  to  the  prevalence  of  sili- 
cate minerals  rich  in  iron,  as  augite,  hornblende,  or  black  mica. 
Rarely  copper,  manganese,  and  other  metallic  oxides  than  those 
of  iron  are  present  in  sufficient  abundance  to  impart  their  char- 
acteristic hues.  As  a  rule,  a  white  or  light  gray  color  denotes 
an  absence  of  an  appreciable  amount  of  iron  in  any  of  its  forms. 
The  amber,  bluish  and  black  colors  of  many  rocks,  particularly 
the  limestones  and  slates,  are  due  to  the  prevalence  of  carbona- 
ceous matter. 

Among  siliceous  crystalline  rocks  the  more  basic,  like  those 
of  the  diabase,  diorite,  or  basalt  groups,  are  as  a  rule  of  a  darker 
color  than  the  acid  varieties,  the  color  being  due  to  the  fine 
grain  and  predominance  of  dark  iron-magnesian  silicates,  such 
as  hornblende,  augite,  or  black  mica,  or  their  chloritic  alteration 
products.  The  red  or  pink  color  sometimes  occurring  in  gran- 
itic rocks  is  due  to  the  predominance  of  red  or  pink  feldspars, 
which  in  their  turn  owe  their  color  to  the  presence  of  iron. 

Among  feldspar-bearing  rocks  the  color  is  not  infrequently 
due  to  the  physical  condition  of  this  important  constituent. 
Thus  in  many  rocks  like  the  norite  of  Keeseville  (New  York), 
and  the  Quincy  (Massachusetts)  granite,  the  dark  color  is 
largely  due  to  the  fact  that  the  feldspar  is  clear  and  glassy, 
allowing  the  light  rays  to  penetrate  and  become  absorbed.  The 
beautiful  chatoyant  play  of  colors  sometimes  shown  by  labra- 
dorite-bearing  rocks  like  those  of  northern  New  York  and  of 
Norway  is  apparently  due  to  a  separation  of  the  individual 
crystals  along  cleavage  lines,  into  thin,  transparent  plates  which 
reflect  and  partially  polarize  the  light  which  would  otherwise 
penetrate  and  become  absorbed.  Through  weathering,  such 
feldspars  undergo  a  further  physical  change,  becoming  soft 
and  porous,  and  no  longer  allowing  the  light  to  penetrate,  but 
wholly  reflecting  it  arid  causing  the  stone  to  appear  white. 
These  white  feldspars,  as  has  been  very  neatly  expressed  by  the 
late  Dr.  Hawes,  bear  the  same  relation  to  the  glassy  forms 
as  does  the  foam  of  the  sea  to  the  water  itself,  the  difference 
in  color  being  in  both  cases  due  to  the  changed  physical  con- 
dition. Indeed,  the  color  of  rocks,  as  may  be  imagined,  is 


THE   COLOR   OF   ROCKS  47 

not  constant,  but  liable  to  change  under  varying  conditions, 
particularly  those  of  exposure.  Rocks  black  with  carbonaceous 
matter  will  fade  to  almost  whiteness  on  prolonged  exposure, 
owing  to  the  bleaching  out  of  the  coloring  materials.  Rocks 
rich  in  magnetite  or  free  iron  oxides,  protoxide  carbonates,  or 
sulphides,  or  in  highly  ferruginous  silicate  minerals,  are  like- 
wise liable  to  a  change  of  color,  becoming  yellowish,  red,  or 
brown,  through  oxidation  of  the  ferruginous  constituents.  (See 
p.  257.)  Translucent,  nearly  colorless  rocks  or  minerals,  as 
those  made  up  of  crystals  of  calcite  or  selenite,  will  on  exposure 
become  nearly  opaque  and  snow-white,  owing  to  purely  physi- 
cal causes,  as  already  noted  in  the  case  of  the  feldspars.  (See 
further  in  chapter  on  weathering.) 

The  cause  of  the  color  variations  in  certain  rocks  and  min- 
erals is,  however,  a  matter  concerning  which  it  will  not  do,  as 
yet,  to  speak  too  decidedly.  Analysis  of  a  mineral  may  show 
the  presence  of  metallic  oxides,  but  it  does  not  necessarily  fol- 
low that  whatever  color  the  mineral  may  have  is  due  or  in  any 
way  related  to  these  oxides.  Thus  the  writer  has  shown 1  that 
the  onyx  marbles  (travertines)  of  Arizona  and  Mexico  may 
vary  from  pure  white  to  green,  and  from  yellow  through  brown 
to  red,  without  appreciable  change  in  the  actual  amounts  of 
iron,  though  there  may  be  a  change  in  the  form  of  combination. 
In  the  white  and  green  varieties  the  iron  exists  as  a  carbonate ; 
in  the  yellow,  red,  and  brown  varieties  as  a  more  or  less  hydrated 
sesquioxide.  Certain  dark  amber  and  bright  rose-colored  va- 
rieties from  California,  and  the  Californian  Peninsula,  show, 
however,  110  iron  or  other  of  the  usual  metallic  coloring  con- 
stituents, but  burn  perfectly  white  when  submitted  to  high 
temperatures  and  yield  volatile  organic  compounds.  The  fact 
that  serpentines  so  frequently  contain  small  traces  of  chromium, 
early  gave  rise  to  the  opinion  that  it  was  to  this  element  that 
was  due  the  characteristic  green  color  of  the  mineral.  The 
writer  has  elsewhere2  described  serpentines  of  a  beautiful  oil 
yellow  and  deep  green  color  which,  however,  contain  not  a 
trace  of  chromium  or  manganese,  but  only  iron,  which  in  this 
case  is  in  combination  as  a  silicate.  (See  p.  114.) 

These  color  characteristics  are  of  greater  importance    than 

1  Annual  Report  U.  S.  National  Museum,  1893,  p.  558. 

2  On  the  Serpentine  of  Montville,  New  Jersey,  Proc.  U.  S.  National  Museum, 
1888,  p.  105. 


48          PHYSICAL  AND   CHEMICAL  PROPERTIES   OF  ROCKS 

may  at  first  appear,  particularly  from  an  economic  standpoint. 
One  of  the  first  essentials  in  a  rock  designed  for  architectural 
use  should  be  permanency  of  color.  Deleterious  changes  are 
particularly  liable  to  occur  in  stone  taken  from  below  the  water 
level,  where,  protected  from  oxidation,  or  from  variations  in 
temperature.  Certain  of  the  Ohio  sandstones  are  of  a  blue- 
gray  color  below  the  water  level,  but  buff  above,  where  the 
included  iron  sulphides  and  protoxide  carbonates  have  been 
acted  upon  by  oxidation.  The  student  should  early  make 
himself  acquainted  with  these  characteristics,  as  in  the  field  it 
is  as  a  rule  only  the  more  or  less  weathered  surfaces  that  pre- 
sent themselves  for  inspection.  This  subject  is  again  referred 
to  in  the  chapter  on  rock  weathering. 

Lustre  as  a  property  of  rocks  does  not,  owing  to  their  com- 
plex nature,  possess  the  same  value  as  a  determinative  charac- 
teristic as  among  minerals.  Certain  of  the  more  compact  and 
homogeneous  varieties  possess  lustres  which  may  be  described 
as  vitreous,  greasy,  pearly,  metallic,  or  iridescent. 

The  meaning  of  such  terms  is  sufficiently  evident,  and  the 
subject  need  not  be  further  dwelt  upon  here.  The  fracture, 
or  manner  of  breaking  of  any  rock,  is  dependent  more  upon 
structure  than  upon  chemical  or  mineralogical  composition. 
Many  fine  and  evenly  grained  crystalline  or  fragmental  rocks 
break  with  smooth,  even  surfaces,  and  are  described  as  having 
a  straight  or  even  fracture.  Others  break  with  shell-like  con- 
cave and  convex  surfaces,  and  are  said  to  have  a  conchoidal 
fracture.  Still  others  are  splintery,  hackly,  or  shaly,  words  the 
meaning  of  which  is  sufficiently  evident  without  their  being 
described  in  detail. 


V.  THE  MODE  OF  OCCURRENCE  OF  ROCKS 

It  is  ordinarily  assumed  that  the  earth  owes  its  present  form 
to  its  having  originated  from  a  mass  of  incandescent  vapor,  and 
to  have  passed,  by  gradual  cooling  and  consequent  condensa- 
tion, from  gaseous  through  pasty  or  fluidal,  and  all  intermediate 
stages,  to  its  present  condition.  This,  in  brief,  is  the  hypothesis 
of  Kant,  and  which  seems  most  readily  to  account  for  the  facts 
as  we  now  know  them.  As  to  the  character  of  the  rock  masses 
resulting  from  this  primary  cooling,  we  know  but  little.  Rea- 
soning from  analogy,  it  seems  safe  to  assume  that  they  resem- 
bled the  slags  from  a  smelting  furnace,  or  some  form  of  modern 
lavas,  more  nearly  than  any  other  rock  masses  of  which  we 
have  knowledge.  Whatever  may  have  been  their  nature,  they 
have  long  since  been  obscured  by  rocks  of  secondary  origin, 
or  become  so  altered  through  dynamic  and  incidental  chemical 
agencies  as  to  be  no  longer  recognizable. 

The  oldest  rocks  of  which  we  now  have  knowledge  belong 
to  the  group  of  gneisses  and  crystalline  schists.  They  are  as 
a  rule  highly  siliceous  rocks,  though  not  infrequently  includ- 
ing considerable  thicknesses  of  crystalline  limestone.  They 
contain  no  traces  of  what  can  be  referred  beyond  doubt  to  an 
organic  origin,  though  from  their  banded  or  foliated  structure, 
so  closely  simulating  bedding,  they  have  in  the  past,  as  a  rule, 
been  considered  metamorphic  rocks  ;  that  is  to  say,  rocks  laid 
down  as  sediments  and  crystallized  by  the  complex  processes 
comprehended  under  the  term  metamorphism.  Rocks  of  this 
type,  according  to  Dana,  first  appeared  in  North  America  in 
the  wide  V-shaped  area  extending  from  Labrador  southwesterly 
to  the  Great  Lake,  and  thence  northwesterly,  to  the  Arctic 
regions.  This  area  has  since  been  added  to  by  the  folding  and 
crumpling  processes  incident  to  the  formation  of  the  Appa- 
lachian and  Rocky  Mountain  systems.  Concerning  the  geo- 
graphical distribution  of  these  rocks,  as  they  now  appear 
exposed,  we  have  little  to  say  here.  They  seem  to  form,  as 
E  49 


50         THE  MODE  OF  OCCURRENCE  OF  ROCKS 

has  been  stated,  the  actual  floor  of  the  continents  upon  which 
all  later  deposits  have  been  laid  down,  and  through  which  and 
into  which  have  been  extruded  and  intruded  the  great  variety 
of  igneous  rocks  which  form  so  conspicuous  a  feature  in  many 
a  mountainous  region.  In  order  to  properly  understand  that 
which  is  to  follow,  we  may  well  devote  a  little  space  here  to  a 
consideration  of  the  manner  in  which  these  rock  masses  occur, 
so  far  as  exposed  to  investigation. 

Several  varieties  of  igneous  rocks,  and  particularly  the  gra- 
nitic types,  occur  not  infrequently  in  the  form  of  immense  oval 
or  rounded  masses,  protruded  into  overlying  materials  which 
dip  away  on  all  sides  ;  such  forms  are  ordinarily  designated 
as  bosses.  (PL  1.)  It  is  a  form  common  to  granite,  gab- 
bros,  norites,  etc.  A  laccolite1  is  a  somewhat  similar  form 
due  to  the  welling  up  of  a  magma  through  a  comparatively 
small  vent,  but  which,  instead  of  coming  to  the  surface,  spread 
out  laterally  into  dome-shaped  masses  between  the  sheets  of  the 
overlying  strata.  When  the  intruded  matter  has  been  so 
forced  into  or  between  overlying  bedded  rocks  as  to  appear 
like  more  or  less  regularly  defined  beds,  they  are  known  as 
sheets  or  sills.  Such,  as  a  rule,  may  be  distinguished  from 
superficial  lava  flows  by  their  like  condition  of  compactness 
along  both  upper  and  lower  contacts,  surface  streams  being 
more  or  less  vesicular  along  the  upper  portions,  owing  to  the 
expansion  of  their  included  moisture.  The  name  dike  is  given 
to  an  eruptive  mass  of  varying  width  included  between  well- 
defined  walls,  and  occupying  a  fissure  or  fault  in  previously 
consolidated  rocks.  Such  are  inclined  at  all  angles  with  the 
horizon,  and  are  usually  of  very  moderate  width,  but  may  ex- 
tend for  miles.  The  dikes  in  any  one  region  will  frequently 
be  found  to  belong  to  one  or  more  well-defined  systems,  each 
system  occupying  fissures  essentially  parallel  with  one  another. 
Any  one  dike  may  remain  comparatively  uniform  in  width  for 
long  distances,  excepting  when  split  up  into  smaller  dikes. 
At  times,  dikes  may  be  traced  to  the  parent  mass  —  a  boss  or 
laccolite  —  from  which  they  radiate  with  more  or  less  regu- 
larity, being  in  such  cases  widest  at  the  start,  and  gradually 

1  It  is  to  be  regretted  that  this  name  in  its  present  form  has  been  so  generally 
adopted  by  geologists,  since  its  termination,  ite,  should  indicate  a  kind  of  rock, 
whereas,  in  fact,  it  but  denotes  a  form  of  occurrence.  Laccolith  would  be 
preferable. 


IGNEOUS   ROCKS  51 

thinning  out  to,  it  may  be,  mere  knife-like  edges.  The  name 
volcanic  neck  or  plug  is  given  to  the  cylindrical  mass  which 
results  from  the  congealing  of  that  portion  of  the  lava  which 
remains  in  the  volcanic  vent  when  eruption  ceases.  Through 
the  erosion  of  the  matter  composing  the  cone  of  a  volcano, 
such  are  sometimes  left,  owing  to  their  superior  hardness,  form- 
ing thus  a  very  striking  feature  of  the  landscape.  The  gen- 
eral name  lava  is  applied  to  any  igneous  rock,  regardless  of 
geological  age  or  mineral  composition,  which  has  been  poured 
out  on  the  surface  of  the  earth  in  a  molten  condition.  Such 
are  characterized,  as  a  rule,  by  less  perfect  crystallization  and  a 
more  slaggy  and  vesicular  structure  than  the  deep-seated  rocks. 
A  columnar  jointing,  due  to  cooling,  is  by  no  means  uncom- 
mon, particularly  among  basaltic  lavas,  although  it  is  by  no 
means  confined  to  them. 

But  a  comparatively  small  proportion  of  the  rocks  composing 
the  superficial  portions  of  the  earth's  crust  —  the  portions  with 
which  we  are  more  or  less  familiar  —  are  eruptive.  They  are 
rather  what  are  known  as  secondary  rocks  ;  that  is  to  say,  they 
are  rocks  made  over  from  these  so-called  primary  rocks,  which 
we  have  been  just  discussing,  by  processes  which  will  be  described 
later. 

Any  rock  mass,  be  it  eruptive  or  otherwise,  lying  exposed  at 
or  near  the  surface  of  the  ground  finds  itself  subjected  to  a 
multitude  of  disintegrating  and  decomposing  agencies,  such  as 
will  be  described  more  in  detail  under  the  head  of  rock  weather- 
ing. Leached  and  decomposed  by  meteoric  waters,  disintegrated 
by  heat  and  frost,  or  the  mechanical  action  of  waves  and  cur- 
rents, the  rock  masses  slowly  succumb,  their  materials  being 
gradually  removed  in  solution,  or  as  debris  mechanically  trans- 
ported by  every  wind,  rain,  or  running  stream,  down  the  slopes 
into  the  valleys,  and  from  the  valleys  into  the  seas.  This 
debris,  in  various  stages  of  coarseness  and  fineness,  and  to 
which  we  give  the  name  of  bowlders,  gravel,  sand,  and  silt, 
undergoes  by  these  transporting  agencies  a  system  of  assorting 
more  or  less  complete,  and  is  carried  to  distances  dependent 
upon  its  weight  and  the  force  of  the  transporting  agent.  It 
requires  no  geological  or  other  special  training  to  enable  one  to 
understand  that  the  force  being  the  same,  the  finer  and  lighter 
materials  will  be  carried  the  farthest,  and  that  all  must  be  de- 
posited when  the  force  shall  be  expended.  Consider,  then,  for 


52         THE  MODE  OF  OCCURRENCE  OF  ROCKS 

purpose  of  illustration,  a  stream  flowing  out  from  a  mountainous 
region  and  emptying  itself  into  a  lake.  Materials  falling  by 
gravity  from  the  mountain  slopes,  or  washed  by  spasmodic  rains 
into  the  stream,  are  transported  certain  distances,  according  to 
the  strength  of  the  current.  For  our  present  purposes,  it  is 
sufficient  to  consider  only  those  portions  which  are  transported 
quite  to  the  mouth  of  the  stream  and  dumped  into  the  lake. 
But  as  the  water  leaves  its  narrow  channel  and  spreads  out  into 
the  lake,  there  is  an  almost  instant  diminution  of  the  force  of 
its  current,  and  consequent  carrying  power.  As  a  result,  it 
begins  to  deposit  its  load,  the  coarsest  and  heaviest  first,  and 
the  finer  materials  further  out  from  the  shore,  the  very  finest, 
an  impalpable  silt  it  may  be,  remaining  suspended  until  the 
very  last.  There  will  thus  be  formed  on  the  bottom  of  the  lake 
or  sea,  whichever  it  may  be,  a  bed,  or  series  of  beds  of  varying 
thickness,  of  gravel,  sand,  and  clay,  the  coarsest  at  the  bottom 
and  nearest  the  shore,  and  the  finest  and  last  the  most  remote. 
But  the  streams  emptying  into  the  lake  vary  from  time  to 
time  in  their  carrying  capacity,  and  the  action  of  the  waves  in 
the  sea  itself,  together  with  the  salts  dissolved  therein,  exert  a 
modifying  action,  whereby  this  process  of  sedimentation,  as  it  is 
called,  may  not  be  quite  so  simple  as  it  first  appears.1  Enough 
has,  however,  been  said  to  show  that  beds  of  detritus  laid  down 
in  this  manner  must  occur  in  approximately  horizontal  layers, 
and  that  the  layers  may  vary  greatly  in  the  coarseness  and 
fineness  of  their  materials,  as  well  as  in  their  mineral  character. 
But  there  are  still  other  processes  of  sedimentation  than  the 
purely  mechanical  methods  described  above.  All  natural  waters 
contain  more  or  less  mineral  matter,  of  which  lime  is  the  more 
abundant.  Through  the  secreting  power  of  marine  animals,  this 
lime  is  taken  up  in  the  form  of  a  carbonate  to  form  shells  and 
calcareous  skeletons  of  molluscs,  corals,  and  other  forms  of 
marine  life.  On  the  death  of  the  secreting  animal,  the  calca- 
reous material  is  left  to  accumulate  in  a  more  or  less  fragmen- 
tal  condition,  forming  thus  the  material  of  the  coral  islands, 
and  to  a  considerable  extent  the  beds  of  limestone  the  world 
over.  I  have  said  to  a  considerable  extent,  for  the  reason  that 
it  is  doubtful  if  many  of  our  limestones  are  of  purely  animal 
origin  ;  in  many  a  true  chemical  precipitation  plays  a  not  unim- 

1  See  Conditions  of  Sedimentary  Deposition,  by  Bailey  Willis,  Journal  of 
Geology,  1893,  p.  476. 


BEDDED   OR   STRATIFIED   ROCKS  53 

portant  part.  This  is  especially  true  of  the  oolitic  varieties, 
and  the  fact  is  readily  apparent  when  we  come  to  study  such 
in  detail.  Consider  a  shallow  sea-bottom  on  which  are  gradu- 
ally accumulating  in  a  finely  divided  condition  the  fragmental 
remains  of  calcareous  organisms  of  any  kind.  By  the  undu- 
latory  action  of  the  waves  these  are  kept  in  almost  constant 
motion,  though  it  may  be  but  gently  rolling  from 'side  to  side. 
Owing  to  evaporation,  or  a  too  rapid  accumulation  of  the  lime 
for  it  to  be  abstracted  by  the  lime-secreting  animals,  the  water 
becomes  supercharged  with  this  constituent,  which  is  then  pre- 
cipitated in  the  form  of  a  thin  pellicle  around  the  most  availa- 
ble nucleus,  in  this  case  the  grains  of  calcareous  sand  upon  the 
bottom.  Thus  are  gradually  built  up  beds  of  no  inconsiderable 
thickness,  such  as  the  well-known  Carboniferous  oolitic  lime- 
stones of  Indiana  and  Kentucky.  The  microscopic  structure  of 
stones  of  this  class  is  shown  in  Fig.  7  on  p.  112.  Rocks  which 
are  laid  down  in  the  manner  we  have  just  described,  whether 
composed  of  inorganic  particles  or  fragmental  materials  from 
marine  and  fresh  water  organisms,  are  designated  as  sediment- 
ary. They  occur  in  more  or  less  well-defined  beds  or  strata, 
and  hence  are  spoken  of  as  bedded  or  stratified.  Owing  to  the 
fact  that  they  have  in  most  cases  been  deposited  in  compara- 
tively shallow  water,  they  retain  not  infrequently  the  superficial 
markings  made  upon  them  by  waves  and  other  agencies  prior  to 
their  final  consolidation. 

Deposits  laid  down  as  above  described  naturally  lie  approxi- 
mately horizontally  where  not  subsequently  disturbed  by  earth 
movements.  The  earth's  crust,  however,  is  by  no  means  in  a 
state  of  stable  equilibrium,  but,  being  subjected  to  continuous 
stress  or  compressive  force,  is  often  broken,  crushed,  or  folded, 
and  crumpled  to  an  extraordinary  degree.  The  name  fault  is 
applied  to  the  profound  fractures  made  by  these  movements, 
and  which,  inclined  at  various  angles  to  the  horizon,  may  extend 
for  miles.  Usually  the  rocks  on  one  side  of  a  fault  will  be 
found  to  have  sunk  down,  while  those  of  the  other  remain  sta- 
tionary or  are  raised,  producing  thus  an  inequality  of  surface 
that  may  assume  mountainous  proportions.  Most  mountain 
ranges,  in  fact,  are  due  to  a  combination  of  faulting  and  fold- 
ing processes.  It  not  infrequently  happens  that  the  masses 
of  rock,  sliding  over  one  another  along  a  line  of  fault,  produce 
smooth  or  striated  and  often  highly  polished  surfaces,  to  which 


54         THE  MODE  OF  OCCURRENCE  OF  ROCKS 

the  name  slickensides  is  given.  Such  are  particularly  noticeable 
among  serpeiitinous  rocks,  being  apparently  due  to  motion  gen- 
erated in  the  mass  by  increase  in  bulk  incident  to  its  conver- 
sion into  serpentine.1  The  name  vein  is  given  to  rock  masses 
of  chemical  origin,  deposited  along  previously  existing  fractures 
which  may  or  may  not  be  true  faults.  By  some  authorities 
the  name  is  also  made  to  include  the  smaller  injections  of  igne- 
ous rocks.  Such  are  here  classed  under  the  head  of  dikes, 
though  it  must  be  understood  that  it  is  not  in  all  cases  pos- 
sible to  state  to  which  of  the  two  classes  an  occurrence  is 
to  be  referred.  It  is  customary  to  divide  the  veins  into  two 
classes :  (1)  the  mineral  veins,  in  which  the  materials  have 
been  deposited  from  aqueous  solution  or  sublimation  between 
the  walls  of  a  fissure ;  and  (2)  segregation  veins,  in  which  the 
component  materials  have  crystallized  or  segregated  out  of  the 
still  unconsolidated,  pasty,  or  colloidal  rock.  It  is  not  in  all 
cases  possible  to  decide  to  which  of  the  two  classes  a  vein  may 
belong,  but  as  a  rule  the  mineral  (or  fissure)  veins  are  separated 
by  sharp  and  well-defined  walls  from  the  country  rock,  and 
show  a  comb  or  banded  structure.  The  segregation  type  is  less 
distinctly  marked,  the  vein  material  being  welded  to  the  enclos- 
ing rock,  or  seemingly  passing  into  it  by  gentle  gradations. 

The  unconsolidated  materials,  as  sands  and  gravels,  occur 
not  only  in  regularly  bedded  or  stratified  forms,  but  also  in 
hillocks  and  ridges  to  which  special  terms  are  applied.  The 
loose  material  washed  down  the  mountain  slopes  by  ephemeral 
streams,  and  deposited  at  the  mouth  of  gorges,  not  infrequently 
assumes  the  form  of  "  a  conical  mass  of  low  slope  descending 
equally  in  all  directions  from  the  point  of  issue."  To  such 
forms  Gilbert  has  given  the  name  of  alluvial  cones.  The  mate- 
rial of  these  cones,  as  described,  varies  in  size  from  the  finest 
powder  to  angular  rocks  weighing  many  tons.  It  exhibits  no 
regular  bedding  or  stratification,  but  coarse  and  fine  debris  are 
mingled  in  endless  variety.  There  is  a  well-marked  gradation, 
however,  to  be  seen  as  one  travels  from  the  apex  of  a  cone 
toward  its  periphery.  At  the  apex  it  is  composed  mostly  of 
coarse,  angular  material,  with  fine  silt-like  clays  filling  the 
interspaces,  while  toward  the  periphery  the  fine  material  pre- 
dominates. The  name  talus  is  given  to  the  accumulations  of 

1  See  On  the  Serpentine  of  Montville,  New  Jersey,  Proc.  U.  S.  National 
Museum,  1888,  p.  105. 


CLASTIC   MATERIALS  55 

debris  at  the  foot  of  rocky  cliffs,  and  which  are  composed  of 
angular  fragments,  large  and  small,  which  have  fallen  from  the 
cliffs  above.  The  name  dune  is  given  to  the  rounded  hills  of 
wind-blown  sand  common  in  arid  regions  and  on  windy  shores. 
Such  are  naturally  of  moderately  fine  and  quite  uniformly 
assorted  materials.  In  form  and  position  they  are  ever  chang- 
ing, like  drifts  of  snow,  but  are  usually  much  steeper  on  the 
leeward  than  on  the  windward  sides.  The  character  of  the 
material  of  which  they  are  composed  is  most  commonly  sili- 
ceous sand. 

The  names  kame,  esker,  osar,  or  horseback  are  given  to  ridges 
and  mounds  of  sand  and  gravel  deposited  by  the  melting  ice  of 
the  glacial  epoch.  The  materials  are  as  a  rule  well  rounded, 
and  as  deposited  usually  show  rude  lines  of  stratification. 
Such,  as  described,  vary  greatly  in  breadth  and  height,  some 
being  400  to  500  feet  broad  at  the  base  and  from  25  to  60  feet 
in  height.  Drumlin  is  the  name  given  to  the  peculiar  low, 
gently  and  smoothly  sloping  lenticular  hills  composed  of  un- 
assorted glacial  debris,  and  which  are  common  in  eastern  Massa- 
chusetts and  other  glacial  regions.  The  general  name  moraine 
includes  the  heterogeneous  materials  brought  down  by  glaciers 
and  ultimately  deposited  in  undulating  hills  and  ridges  on 
their  final  disappearance.  (See  further  under  The  Regolith, 
p.  299.) 


PART    II 

THE  KINDS  OP  BOCKS 

"  Some  rin  up  hill  and  down  dale  knapping  the  chucky  stones  to  pieces  wi 
hammers  like  sae  many  road-makers  run  daft.  They  say  it  is  to  see  how  the 
•warld  was  made."  —  St.  Ronan's  Well. 

REFERENCE  has  already  been  made  to  the  fact  that  but 
sixteen  out  of  the  sixty-nine  known  elements  enter  into  the 
composition  of  the  earth's  crust  in  other  than  comparatively 
minute  quantities.  Also  to  the  equally  important  fact  that  the 
combination  of  these  elements  as  represented  in  not  above  a 
score  of  well-known  mineral  species  go  to  make  up  the  essential 
portion  of  nearly  all  rock  masses.  Nevertheless,  owing  to  the 
variety  of  forms  under  which  these  rock  masses  occur,  the  vary- 
ing forces  or  conditions  under  which  they  originated,  or  the 
proportional  quantities  of  the  various  minerals  which  they  may 
contain,  we  find  numerous  and  widely  varying  types  of  rocks, 
a  satisfactory  consideration  of  which  necessitates  first  some 
attempt  at  systematic  classification.  We  may  say  at  the  outset, 
however,  that  rock  species,  in  the  sense  in  which  the  word  is 
o  used  in  mineralogy  and  zoology,  scarcely  exist.  It  is  true  we 
may  have,  and  particularly  among  igneous  rocks,  certain  forms 
which  on  casual  inspection,  or  indeed  on  close  inspection,  with 
regard  only  to  limited  geographical  areas,  seem  to  possess  an 
individuality  of  their  own  sufficient  to  entitle  them  to  being 
considered  as  true  species.  Yet,  when  we  come  to  compare 
these  with  others,  to  take  into  account  their  physical  and  chem- 
ical composition,  their  structure  and  mode  of  occurrence,  and 
above  all  to  consider  how  any  rock  varies  within  its  own  mass, 
and  the  still  greater  variation  which  may  have  been  produced 
through  alteration,  we  shall  see  that  one  form  grades  into  an- 
other almost  without  limit,  that,  indeed,  no  two  are  exactly 
alike,  and  that,  were  we  to  attempt  any  hard  and  sharp  lines  of 
discrimination,  our  species-making  would  practically  resolve  it- 

56 


THE   KINDS   OF   ROCKS  57 

self  into  an  enumeration  of  individual  occurrences,  or  specimens. 
This  fact  will  become  apparent  as  we  proceed,  and  further 
remarks  on  the  subject  may  well  be  deferred  until  we  come  to 
a  discussion  of  individual  groups.  Indeed,  in  the  present,  tran- 
sitional state  of  knowledge  regarding  the  chemical  and  minera- 
logical  composition  of  rocks,  their  structural  features,  and 
methods  of  origin,  no  scheme  of  classification  can  be  advanced 
that  will  prove  satisfactory  in  all  its  details.  The  older  sys- 
tems, which  were  made  to  answer  before  the  introduction  of  the 
microscope  into  geological  science,  are  now  known  to  be  founded 
upon  what  were  in  part  false,  and  what  have  proven  to  be 
wholly  inadequate,  data.  This  is  especially  true  in  regard  to 
eruptive  rocks.  The  time  that  has  elapsed  since  this  intro- 
duction has  been  too  short  for  the  evolution  of  a  perfectly  satis- 
factory system  ;  many  have  been  proposed,  but  all  have  been 
found  lacking  in  some  essential  particulars.  To  enter  upon  a 
discussion  of  the  merits  and  demerits  of  the  various  schemes 
would  obviously  be  out  of  place  here,  and  the  student  is  re- 
ferred to  the  published  writings  of  Naumann,  Senft,  Von  Cotta, 
Richtofen,  Vogelsang,  Zirkel,  Rosenbusch,  Michel-Levy,  Cred- 
ner,  Jukes  Brown,  and  Geikie,  as  well  as  those  of  the  American 
geologists,  Dana,1  Wadsworth,2  and  Iddings.3  In  the  scheme 
here  presented  the  writer  has  aimed  to  simplify  matters  so  far 
as  is  consistent  With  observed  facts,  and  has  not  hesitated  to 
adopt  or  reject  any  such  portions  of  systems  proposed  by  others 
as  have  seemed  desirable. 

All  the  rocks  forming  any  essential  part  of  the  earth's  crust 
are  here  grouped  under  four  main  heads,  the  distinctions  being 
based  upon  their  origin  and  structure.  Each  of  the  main 
divisions  is  again  divided  into  groups  or  families,  the  distinc- 
tions being  based  mainly  upon  mineral  and  chemical  composi- 
tion, structure,  and  mode  of  occurrence.  We  thus  have  :  — 

I.  Igneous  Rocks :  Eruptive.  —  Rocks  which  have  been 
brought  up  from  below  in  a  molten  condition,  and  which 
owe  their  present  structural  peculiarities  to  variations  in  con- 
ditions of  solidification  and  composition.  Having  as  a  rule  two 

1  On  Some  Points  in  Lithology,  Am.  Jour,  of  Science,  Vol.  XVI,  1878,  pp.  335 
and  431. 

2  On  the  Classification  of  Rocks,  Bull.  Mus.  Comp.  Zool.  Howard  College, 
No.  13,  Vol.  V  ;  also  Lithological  Studies. 

3  The  Origin  of  Igneous  Rocks,  Bull.  Philosophical  Society  of  Washington, 
1892. 


58  THE   KINDS  OF   ROCKS 

or  more  essential  constituents.     In  structure  massive,  crystal- 
line, or  glassy,  or  in  certain  altered  forms,  colloidal. 

II.  Aqueous    Rocks.  —  Rocks   formed   mainly   through   the 
agency  of  water,  as  (J.)  chemical  precipitates  or  as  (#)  sedi- 
mentary beds.     Having  one  or  many  essential  constituents.     In 
structure  laminated  or  bedded ;  crystalline,  colloidal,  or  f  rag- 
mental  ;  never  glassy. 

III.  JEolia.n  Rocks. — Rocks  formed  from  wind-drifted  ma- 
terials.    In  structure  irregularly  bedded  ;  fragmental. 

IV.  Metamorphic  Rocks.  —  Rocks  changed  from  their  orig- 
inal   condition    through   dynamic    or   chemical   agencies   and 
which  may  have  been  in  part  of  aqueous,  seolian,  or  of  igne- 
ous origin.      Having  one  or  many  essential  constituents.     In 
structure  bedded,  schistose  or  foliated,  and  crystalline. 


I.    ROCKS   FORMED    THROUGH   IGNEOUS 
AGENCIES.     ERUPTIVE 

This  group  includes  all  those  rocks  which  having  once  been 
in  a  state  of  igneous  fusion  have  been  forced  upward  and  in- 
truded into  the  overlying  rocks  in  the  form  of  bosses,  laccolites, 
dikes,  and  sheets,  or  poured  out  upon  the  surface  as  lavas. 

Concerning  the  source  of  eruptive  rocks  we  are  yet  in  igno- 
rance. By  many  they  have  been  supposed  to  represent  portions 
of  the  still  unconsolidated  interior  of  the  earth.  The  great 
variety  of  igneous  rocks,  the  wide  variation  in  chemical  compo- 
sition as  well  as  the  apparent  independence  of  closely  adjacent 
volcanoes,  both  in  the  matters  of  time  of  eruption  and  character 
of  erupted  material,  seem,  however,  to  show  that  they  come  not 
from  a  common  reservoir,  but  from  isolated  and  comparatively 
small  areas  where,  for  reasons  not  now  well  understood,  pre- 
viously solidified  rock  masses  have  been  so  highly  heated  as  to 
become  pasty  or  liquid  ;  and  then,  through  their  own  expan- 
sion, or  that  of  included  vapors,  or  by  compressive  forces 
generated  in  the  earth's  crust,  forced  upward  into  the  positions 
they  now  occupy.  The  origin  of  igneous  rocks  belongs  as  yet 
largely  to  the  realm  of  speculation.  We  must  here  confine 
ourselves  more  to  their  mineral  and  chemical  nature,  general 
physical  properties,  and  the  conditions  under  which  they  occur. 

Consider,  then,  a  mass  of  molten  rock  material,  —  to  which 
the  term  magma  may  be  conveniently  applied,  —  and  which  by 
the  processes  of  eruption  is  forced  upward  toward  the  surface, 
and  let  us  first  dwell  briefly  upon  the  forms  assumed  by  this 
magma  on  cooling  under  the  various  conditions  in  which  it 
finds  itself.  It  is  obvious  at  the  start  that  we  can  have  actu- 
ally to  do  with  but  a  comparatively  limited  portion  of  the 
products  of  any  eruption.  If  the  molten  material  is  poured 
out  upon  the  surface  and  there  remains  for  our  inspection 
to-day,  it  is  a  necessary  consequence  that  the  deeper-lying 
portions  are  obscured.  If,  on  the  other  hand,  the  superficial 

59 


60  ROCKS   FORMED  THROUGH  IGNEOUS  AGENCIES 

portions  have  been  removed  by  erosion  so  as  to  expose  the 
deeply  lying  parts,  we  have  only  these  for  study  and  observa- 
tion. It  is  rare  indeed  that  erosion  has  so  acted  on  any  one 
rock  mass  as  to  expose  superficial  and  deep-seated  portions 
alike.  In  the  older  regions,  —  those  of  greatest  geological  an- 
tiquity,—  erosion,  either  glacial  or  otherwise,  has  not  infre- 
quently removed  more  or  less  completely  the  superficial  parts 
and  left  for  our  inspection  those  portions  of  a  magma  that  at 
the  time  of  eruption  never  reached  the  surface,  but  cooled,  it 
may  be,  under  thousands  of  feet  of  superincumbent  matter. 
Such  rocks  are  as  a  rule  more  highly  crystalline  than  those 
which  in  the  newer,  less  eroded  portions,  flowed  out  upon  the 
surface  like  our  modern  lavas.  Hence  it  is  that  from  a  very 
early  period  it  has  been  found  convenient,  for  purposes  of  dis- 
cussion, to  divide  the  eruptive  rocks  into  two  general  groups : 
first,  the  intrusive  or  plutonic  rocks  ;  and  second,  the  effusive, 
or  volcanic  rocks. 

Although  this  classification  has  not  been  strictly  adhered  to 
in  the  present  work,  a  few  words  descriptive  of  the  essential 
distinctions  between  plutonic  and  effusive  rocks  will  not  be  out 
of  place,  since  such  distinctions,  particularly  in  eroded  regions, 
afford  the  only  criteria  for  discrimination  as  to  the  original 
conditions  under  which  a  rock  mass  has  been  formed,  and  hence 
are  of  value  in  the  field. 

As  a  general  rule,  it  may  be  said  that  the  structural  features 
of  an  eruptive  rock  depend  upon  the  conditions  under  which 
a  magma  has  cooled,  although  undoubtedly  the  amount  of 
included  vapor  of  water  may  exert  a  powerful  influence.  As 
Professor  J.  P.  Iddings  has  well  expressed  it,  uthe  chemical 
differences  of  igneous  rocks  are  the  result  of  a  chemical  differ-- 
entiation  of  a  general  magma,  and  the  structure  of  a  rock  is 
dependent  upon  the  physical  conditions  attending  its  eruption 
and  solidification."  Now  it  is  at  once  apparent  that  the  greater 
the  depth  below  the  surface  at  which  a  magma  'undergoes 
solidification,  or  the  greater  its  mass,  the  slower,  more  gradual, 
will  be  that  solidification,  and  hence  the  more  complete  and 
coarser  will  be  the  crystallization.  Hence  the  strictly  plutonic 
rocks  are  always  holocrystalline.  And,  inasmuch  as  the  weight 
of  the  superincumbent  matter  has  been  such  as  to  prevent  the 
expansion  of  included  vapors  to  form  steam  cavities,  so  these 
rocks  are  never  vesicular  or  pumiceous,  but  compact  and  gran- 


STRUCTURAL   FEATURES   OF  IGNEOUS   ROCKS  61 

ular  throughout.  In  cases  where  a  plutonic  rock  has  been 
voided  upward  to  fill  a  pre-existing  rift  in  the  form  of  a  dike, 
those  portions  of  the  magma  coming  in  contact  with  the  cold 
walls  on  either  hand  will  cool  most  quickly.  Hence  a  dike  is 
as  a  rule  most  coarsely  crystalline  near  the  centre,  becoming 
finer  grained  and  perhaps  microcrystalline  or  even  glassy  at 
the  immediate  contact.  These  two  phenomena  often  afford 
the  only  means  of  determining  whether  a  rock  mass  occurring 
in  the  form  of  a  sheet  parallel  with  the  stratification,  between 
sedimentary  beds,  is  an  intrusive  or  a  contemporaneous  lava 
flow ;  whether  it  was  injected  as  we  now  find  it  between 
two  previously  existing  beds  ;  or  whether,  as  a  lava  flow,  it 
was  poured  out  over  the  lower,  first  formed,  after  which  the 
second  was  laid  down  upon  its  surface.  If  formed  as  an  intru- 
sive sheet,  we  may  expect  to  find  the  rock  more  dense  along 
both  contacts,  in  addition  to  which  there  may,  very  probably, 
be  more  or  less  contact  metamorphism  on  the  sedimentary  beds 
from  the  action  of  the  hot  intruded  material.  If  poured  out 
as  a  lava,  on  the  other  hand,  contact  metamorphism  and  the 
dense,  fine-grained  portions  will  be  limited  to  the  lower  con- 
tacts, while,  provided  there  had  been  110  great  amount  of  erosion 
between  the  time  of  the  pouring  out  of  the  molten  mass  as  a 
surface  flow  and  the  deposition  of  the  newer  sediments,  the 
upper  portions  will  be  less  dense,  perhaps  even  vesicular,  sco- 
riaceous,  and  glassy,  while  the  sediments  themselves,  having 
been  laid  down  on  cold  consolidated  material,  remain  wholly 
unchanged.  Such  means  of  discrimination  have  been  of  the 
greatest  value  in  ascertaining  the  relative  ages  of  portions 
of  the  Triassic  sandstones  and  associated  traps  in  the  eastern 
United  States. 

The  lava  flows,  cooling  so  much  more  rapidly  than  the  plu- 
tonic rocks,  owing  to  their  exposed  position  and  relief  from 
pressure,  often  show  but  incipient  forms  of  crystallization,  or 
are  quite  glasslike,  as  is  the  case  with  the  obsidians  of  the 
Yellowstone  Park  and  elsewhere.  Chemically  these  are  iden- 
tical with  granite,  but  they  have  cooled  too  quickly  for  the 
forces  of  crystallization  to  act.  Owing,  further,  to  the  expan- 
sive force  of  the  included  vapor  of  water,  —  a  constituent  of 
all  lavas,  —  these  surface  flows  are  not  infrequently  so  filled 
with  cavities  as  to  be  quite  pumiceous.  The  pumice  pur- 
chased at  the  drug-stores  is  but  the  froth  from  a  lava  which, 


62  ROCKS  FORMED  THROUGH   IGNEOUS  AGENCIES 

had  it  cooled  under  greater  pressure,  might  have  given  us  a 
granite. 

A  common  feature  of  the  effusive  or  volcanic  rocks  is  a  flow 
structure,  sometimes  visible  only  with  the  microscope,  and  which 
is  due  to  a  flowing  movement  of  the  magma  while  undergoing 
consolidation.  (See  Fig.  2,  PI.  2.)  The  characteristic  structure 
of  effusive  rocks  is  porphyritic,  instead  of  granular,  and  repre- 
sents two  distinct  phases  of  cooling  and  crystallization  :  (1)  an 
intratellurial  period,  marked  by  the  crystallization  of  certain 
constituents  while  the  magma,  still  buried  in  the  depths  of  the 
earth,  was  cooling  very  gradually,  and  (2)  an  effusive  period, 
marked  by  the  final  consolidation  of  the  material  on  or  near 
the  surface.  As  this  final  cooling  was  much  the  more  rapid, 
the  ultimate  product  is  a  glassy,  felsitic,  or  sometimes  holo- 
crystalline  ground-mass,  enclosing  the  porphyritic  minerals,  or 
phenocrysts,  formed  during  the  first  or  intratellurial  stage.1 
Naturally  the  deeper-lying  portions  of  an  effusive  mass,  those 
forming  the  under  or  lower  portions  of  deep  lava  streams,  will 
be  under  conditions  essentially  similar  to  plutonic  magmas,  and 
may  cool  so  slowly  as  to  become  holocrystalline.  It  is,  more- 
over, obvious  that,  could  we  trace  any  superficial  mass  of 
erupted  material  back  to  its  original  deep-seated  source,  we 
would  pass  gradually  from  the  volcanic  to  the  plutonic  type 
without  at  any  one  point  being  able  to  indicate  the  line  of 
separation.  Hence  it  is  that  in  the  laboratory  it  is  not  always 
possible,  from  the  examination  of  the  hand  specimen  or  thin 
section  only,  to  determine  to  which  of  the  two  classes  it  may 
belong.  We  can  easily  discriminate  between  the  extremes, 
but  there  is  a  wide  intermediate  zone  where  any  such  attempts 
are  impracticable,  as  indeed  they  are  unnecessary.2 

1  Whitman  Cross  has  shown  that  there  are  exceptions  to  this  rule.    See  The 
Laccolitic  Mountain  Groups  of  Colorado,  14th  Ann.  Rep.  U.  S.  Geol.  Survey, 
pp.  231-235. 

2  Intermediate  between  these  plutonic  and  effusive  types  is  still  a  third  phase 
of  prevailing  holocrystalline  porphyritic  structure,  and  which,  owing  to  the  fact 
that  such  have  thus  far  been  found  only  in  dikes,  it  has  been  proposed  to  group 
under  the  head  of  dike  rocks  (gangesteine).     Since  such  are  but  local  phases  of 
plutonic  magmas,  which  have  been  left  to  cool  and  crystallize  between  narrow 
walls,  instead  of  poured  out  upon  the  surface,  such  a  subdivision  seems  scarcely 
called  for  and  as  tending  to  still  further  confuse  that  which  is  already  sadly 
confounded.    The  same  may  be  said  with  reference  to  the  now  prevailing  ten- 
dency to  give  varietal  names  to  every  phase  of  magmatic  differentiation,  and 
which  has  resulted  already  in  such  monstrosities  of  nomenclature  as  ouachitite, 
monchiquite,  yogoite,  and  absarokite. 


RELATIONSHIP   OF    PLUTONIC   AND   IGNEOUS   ROCKS         63 

Owing  to  a  false  impression  which  formerly  prevailed  relative 
to  the  nature  of  the  Palaeozoic  effusives  and  those  of  Mesozoic, 
Tertiary,  and  more  recent  times,  dissimilar  names  have,  in  very 
many  instances,  been  applied  to  rocks  which  in  other  respects 
than  that  of  geological  age  are  essentially  one  and  the  same. 
Thus  the  name  andesite  is  given  to  a  rock  in  every  respect 
similar  to  porphyrite,  with  the  possible  exception  of  a  slight 
amount  of  devitrification  the  latter  may  have  undergone  owing 
to  its  greater  geological  antiquity. 

The  name  rhyolite  likewise  includes  rocks  with  the  structure 
and  composition  of  the  older  quartz  porphyries,  and  though 
intended  by  Richthofen  to  include  only  certain  comparatively 
modern  acid  lavas,  has  been  shown  by  the  late  Dr.  Williams l 
to  be  equally  applicable  to  the  pre-Cambrian  lavas  of  the  South 
Mountain  region  of  Pennsylvania.  These  and  other  names  have, 
however,  become  too  firmly  engrafted  upon  the  literature  to  be 
too  hastily  set  aside,  and  may  well  be  retained  here. 

The  following  table  will  serve  to  show  the  relationship,  so 


INTRUSIVE  OB  PLUTONIC 

EFFUSIVE  OR  VOLCANIC 

Palseovolcanic 

Neovolcanic 

Acid 

65%  -75%     }•  Granites  .... 

Quartz  porphyries  .     . 

Liparites  (rhyolites) 

Si02         J 

r  Syenites   .... 
Intermediate      N     heline  syenitesj 

00%  to  65%          PFoyai4 

Quartz-free  porphyries 
Phonolites  

Trachytes 
Phonolites 

bl°2           iDiorites    .     .     .     . 

Porphy  rites    .... 

Andesites 

f  Gabbros,    norites,  ( 

Melaphyrs  and  augitej 

"Rooqlfo 

and  diabases        \ 

porphyrites                \ 

Basic 

Theralites     .     .     . 

(Not  known)  .... 

(Thephrites    and 
|     basanites 

40%  to  55%    H 

Peridotites    .     . 

Picrite  porphyrites 

Limburgites 

Si02 

Pyroxenites  . 

(Not  known)      .     .     . 

Augitites 

(Not  known)     .     . 

(Not  known)      .     .     . 

Leucite  rocks 

(Not  known) 

(Not  known)      .     .     . 

Nepheline  rocks 

(Not  known)     .     . 

(Not  known)      .     .     . 

Melilite  rocks 

far  as  known,   which   exists  between  the  plutoiiic  rocks   and 
their  effusive  equivalents  of  whatever  age.     Thus  the  palseo- 


Am.  Jour,  of  Science,  Vol.  XLIV,  p.  482,  1892. 


64  ROCKS  FORMED  THROUGH   IGNEOUS  AGENCIES 

volcanic  equivalents  of  the  syenites  are  the  quartz-free  por- 
phyries, and  the  neovolcanic  equivalents,  the  trachytes.  The 
terms  acid,  intermediate,  and  basic,  as  used,  have  reference  to 
the  percentage  amounts  of  silica,  both  free  and  combined, 
contained  by  the  representatives  of  the  several  groups.  Rocks 
which,  like  some  of  the  peridotites,  carry  even  less  than  40  % 
of  silica  are  sometimes  spoken  of  as  ultra  basic. 

The  researches  of  the  past  few  years  have  made  it  apparently 
evident  that*  eruptive  rocks  are  to  be  satisfactorily  studied  only 
when  considered,  in  their  geographical  as  well  as  geological 
relationships  ;  that  is  to  say,  the  eruptives  of  any  particular 
region  must  be  considered  with  reference  to  their  genetic  rela- 
tion to  others  of  the  same  region  ;  such  a  relationship  as  is 
suggested  by  regarding  them  all  as  but  varying  phases  of  a 
process  of  differentiation  from  a  common  magma. 

That  such  a  relationship  in  many  cases  exists  has  apparently 
been  conclusively  demonstrated  by  the  work  of  Iddings 1  in  the 
Yellowstone  Park,  J.  F.  Williams2  in  Arkansas,  Pirsson3  in 
Montana,  and  Brogger4  in  Norway.  The  attempt  at  correla- 
tion of  local  types  with  those  of  a  somewhat  similar  nature  at 
a  distance  is  interesting  and  instructive,  as  showing  on  the 
whole  a  remarkable  unity  in  nature's  methods  ;  but  we  must 
never  lose  sight  of  the  fact  that  each  eruptive  centre,  through- 
out periods  of  activity  interrupted  it  muy  be  by  thousands  of 
years,  works  out  its  own  results  according  to  local  conditions 
which  may  or  may  not  harmonize  with  those  at  distant  points. 
It  is  possible  to  conceive  that,  could  all  the  rocks  of  any  suc- 
cessive periods  of  eruption  from  a  single  centre  be  once  more 
relegated  to  a  common  magma,  such  might,  in  its  entirety,  be 
an  exact  equivalent  of  others  in  remote  portions  of  the  globe. 
The  consolidated  results  from  the  cooling  of  extruded  portions 
of  this  magma  may,  however,  show  ever-varying  differences 
due  to  local  conditions.  In  short,  eruptive  rocks  must  be 
considered  by  geographic  groups  and  with  reference  to  magmas. 

Attempts  at  a  satisfactory  classification  on  other  grounds 
must  prove  invariably  futile  and  tend  only  to  retard,  rather 
than  to  promote,  the  science. 

1  Bull.  Philos.  Soc.  of  Washington,  XII,  1892. 

2  Ann.  Rep.  Geol.  Survey  of  Arkansas,  Vol.  II,  1890. 

3  Bull.  Geol.  Soc.  of  America,  Vol.  VI,  1895. 

4  Die  Eruptivgesteine  des  Kristianiagebiete,  Christiana,  Norway,  1894. 


PLATE   6 


*tt«f!. 


'S 

•  •/ V  *  •*.'>/r'  -  ,  ^'  >  *^    *  •" 

•-*- 


FIG.  1.  Lithophysse  in  liparite. 
FIG.  2.   Cross-section  of  stalagmite. 


FIG.  3.   Concretionary  aragonite. 
FIG.  4.   Pegmatite. 


THE   GRANITE-LIPARITE   GROUP 


65 


In  the  following  pages  the  rocks  are  discussed  in  groups, 
each  group  comprising  all  those  rocks  having  essentially  the 
same  chemical  composition,  but  differing  (1)  in  degree  of 
crystallization,  (2)  in  mode  of  occurrence,  and  (3)  in  geological 
age.  In  all,  there  is,  within  certain  limits,  a  considerable  range 
in  mineral  composition,  or  at  least  in  the  relative  proportion  of 
the  various  essential  constituents. 

1.     THE  GRANITE-LIPARITE  GROUP 

This  group  includes  the  most  acid  of  all  eruptive  rocks ;  that 
is,  those  which  on  analysis  are  found  to  yield  the  highest  per- 
centages of  silica.  Their  chief  essential  constituents  are  quartz 
and  potash  feldspars,  while  the  more  basic  ferruginous  minerals 
are  in  quantities  proportionately  small.  The  group  includes  a 
deep-seated  or  plutonic  type,  granite,  and  two  effusive  or  vol- 
canic types,  quartz  porphyry,  and  liparite  or  rhyolite.  They 
may  be  described  in  detail  as  below  :  — 

(1)  THE   GRANITES 

Granite,  from  the  Latin  "  granum,"  a  grain,  in  allusion  to  the 
granular  structure. 

Mineral  Composition.  —  The  essential  constituents  of  granite 
are  quartz  and  a  potash  feldspar  (either  orthoclase  or  micro- 
cline),  and  plagioclase.  Nearly  always  one  or  more  minerals  of 
the  mica,  hornblende,  or  pyroxene  group  are  present,  and  in 
small,  usually  microscopic  forms,  the  accessories  magnetite, 
apatite,  and  zircon  ;  more  rarely  occur  sphene,  beryl,  topaz, 
tourmaline,  garnet,  epidote,  allanite,  fluorite,  and  pyrite.  De- 
lesse1  has  made  the  following  determination  of  the  relative 
proportion  of  the  various  constituents  in  two  well-known  gran- 
ites :  — 


EGYPTIAN  RED  GRANITE 

PARTS 

PORPIIYRITIC  GRANITE,  VOSGES 

PARTS 

R6d  orthoclase 

43 

White  ortlioclase 

28 

White  albite    .... 

9 

Reddish  oligoclase      .... 

7 

Gray  quartz 

44 

Gray  quartz   

59 

Black  mica  . 

4 

Mica                         

6 

Total     

100 

Total  

100 

1  Prestwich,  Chemical  and  Physical  Geology,  Vol.  I,  p.  42. 


66 


ROCKS  FORMED   THROUGH  IGNEOUS   AGENCIES 


Chemical  Composition.  —  A  general  idea  of  the  varying  char- 
acter of  these  rocks  may  be  gained  from  the  following  analy- 
ses :  — 


KINDS  AND  LOCALITIES 

Si02 

A1S0, 

FeO 
Fe203 

CaO 

MgO 

K20 

NaaO 

Biotite  granite,  near  Dublin, 
Ireland        

73.0 

13.64 

2.44 

1.84 

2.11 

4.21 

3.53 

Biotite  granite,  Silesia      .     . 
Biotite      granite,      Raleigh, 
North  Carolina    .... 
Hornblende     granite,      Salt 
Lake    Utah 

73.13 
69.28 
71  78 

12.49 
17.44 
14  75 

2.58 
2.30 
1  94  * 

2.40 
2.30 
2  36 

0.27 
0.27 
0  71 

4.13 

2.76 
4.89 

2.61 
3.64 
3  12 

Hornblende     granite,     Sauk 
Rapids,  Minnesota  .     .     . 
Gneissoid     biotite     granite, 
District  of  Columbia    .     . 
Hornblende     mica    granite, 
Svene    EffVDt 

64.13 
69.33 

68.18 

21 
14.33 

16.20 

01 
3.60 
4.10 

6.90 
3.21 
1.75 

1.26 
2.44 
0.48 

1.22 
2.67 
6.48 

3.31 
2.70 

288 

Although  the  mineral  apatite  is  so  universally  a  constituent 
of  granitic  rocks,  yet  it  occurs  in  such  small  quantities  as  to 
be  quite  overlooked  in  the  ordinary  methods  of  analysis.  Such 
tests  as  have  been  made  show  that  the  amount  of  phosphoric 
acid  (P2O5)  contained  by  rocks  of  this  class  rarely  exceeds 
0.2  %  and  may  fall  as  low  as  0.05  %.  Small  as  is  the  amount, 
it  is  nevertheless  probable  that  it  was  from  just  such  minute 
quantities  in  granites  and  the  more  basic  eruptives,  that  was 
derived  the  main  supply  of  phosphates  existing  in  soils. 

Structure. —  The  granites  are  holocrystalline  granular  rocks. 
As  a  rule  none  of  the  essential  constituents  show  perfect  crystal 
outlines,  though  the  f eldspathic  minerals  are  often  quite  perfectly 
formed.  The  quartz  has  always  been  the  last  mineral  to  so- 
lidify, and  hence  occurs  only  as  irregular  granules  occupying  the 
interspaces.  It  is  remarkable  from  its  carrying  innumerable 
cavities  filled  with  liquid  and  gaseous  carbonic  acid  or  with 
saline  matter.  So  minute  are  these  cavities  that  it  has  been  esti- 
mated by  Sorby  that  from  one  to  ten  thousand  millions  could 
be  contained  in  a  single  cubic  inch  of  space.  The  microscopic 
structure  of  a  mica  granite  from  Maine  is  shown  in  Fig.  3  and 
in  Fig.  1,  PI.  5. 


1  Yielded  also  1.09%  manganese  oxide. 


THE   GRANITE-LIP  ARITE   GROUP 


67 


FIG.  3.  —  Microstructure  of  muscovite-biotite 
granite,  Hallowell,  Maiue. 


The  rocks  vary  in  texture  almost  indefinitely,  presenting  all 
gradations  from  fine  evenly  granular  rocks  to  coarsely  porphy- 
ritic  forms  in  which  the 
feldspars,  which  are  the 
only  constituents  porphy- 
ritically    developed,     are 
several  inches  or  feet  in 
length. 

Concretionary  forms 
are  rare.  A  variety  from 
Craftsburg,  Vermont,  is 
unique  on  account  of  the 
numerous  concretionary 
masses  of  black  mica  it 
carries. 

Colors.  —  The  prevail- 
ing color  is  some  shade 
of  gray,  though  greenish, 
yellowish,  pink,  to  deep 
red,  are  not  uncommon. 
The  various  hues  are  due  to  the  color  of  the  prevailing  feldspar 
and  the  abundance  and  kind  of  the  accessory  minerals.  Granites 
in  which  muscovite  is  the  prevailing  mica,  are  nearly  always  very 
light  gray  in  color.  The  dark  gray  varieties  are  due  largely  to 
abundant  black  mica  or  hornblende,  the  greenish  and  pink  or 
red  colors  to  the  prevailing  greenish,  pink,  or  red  feldspars. 

Classification  and  Nomenclature.  —  Several  varieties  are  com- 
monly recognized  and  designated  by  names  dependent  upon  the 
predominating  accessory  mineral.  We  thus  have  (1)  musco- 
vite granite,  (2)  biotite  granite  or  granitite,  (3)  biotite-muscovite 
granite,  (4)  hornblende  granite,  (5)  Jiornblende-biotite  granite,  and 
more  rarely  (6)  pyroxene  (7)  tourmaline  and  (8)  epidote  granite. 
The  name  protogine  has  been  given  to  a  granite  in  which  the 
mica  is  in  part  or  wholly  replaced  by  talc.  The  name  is  not 
very  generally  used. 

G-raphic  granite,  or  pegmatite,  is  a  granitic  rock  consisting 
essentially  of  quartz  and  orthoclase  so  crystallized  together  in 
long  parallel  columns  or  shells  that  a  cross-section  bears  a 
crude  resemblance  to  Hebrew  writing.  (See  Fig.  4,  PI.  6.) 
Aplit  is  a  name  used  by  the  Germans  for  a  granite  very  poor  in 
mica  and  consisting  essentially  of  quartz  and  feldspar  only. 


68  HOCKS   FORMED   THROUGH   IGNEOUS   AGENCIES 

The  names  granitell  and  Unary  granite  have  also  been  used 
to  designate  rocks  of  this  class.  Ghreisen  is  a  name  applied  to 
a  quartz-mica  rock,  with  accessory  topaz,  occurring  associated 
with  the  tin  ores  of  Saxony  and  regarded  as  a  granite  meta- 
morphosed by  exhalations  of  fluoric  acid.  Luxullianite  and 
Trowlesworthite  are  local  names  given  to  tourmaline  or  tour- 
maline-fluorite  granitic  rocks  occurring  at  Luxullian  and 
Trowlesworth,  in  Cornwall,  England.  The  name  Unakite  has 
been  given  to  an  epidotic  granite  with  pink  feldspars  occurring 
in  the  Unaka  Mountains  in  western  North  Carolina  and  eastern 
Tennessee. 

The  name  granite  porphyry  is  made  to  include  a  class  of  rocks 
placed  by  Professor  Rosenbusch  under  the  head  of  "gaiige- 
steine,"  or  dike  rocks,  and  differing  from  the  true  granites 
mainly  in  structural  features.  They  consist  in  their  typical 
forms  of  orthoclase  feldspars  and  quartzes  porphyritically  de- 
veloped in  a  finer  holocrystalline  aggregate  of  the  minerals 
common  to  the  granite  group. 

The  granites  are  among  the  most  wide-spread  and  commonest 
of  rocks,  and  are  of  great  economic  importance  for  structural 
and  monumental  work.  In  the  United  States  they  are  to  be 
found  mainly  in  the  Appalachian  region  and  from  the  front 
range  of  the  Rocky  Mountains  westward  to  the  Pacific  coast. 

Geological  Age  and  Mode  of  Occurrence.  —  The  granites  are 
massive  rocks,  occurring  most  frequently  associated  with  the 
older  and  lower  rocks  of  the  earth's  crust,  sometimes  inter- 
stratified  with  metamorphic  rocks  or  forming  the  central  por- 
tions of  mountain  chains.  They  are  not,  as  once  supposed,  the 
oldest  of  rocks,  but  occur  frequently  in  eruptive  masses  or 
bosses  invading  rocks  of  all  ages  up  to  late  Mesozoic  or  Ter- 
tiary times.  Thus  Professor  Whitney  considered  the  eruptive 
granites  of  the  Sierra  Nevada  to  be  Jurassic.  Zirkel  divides 
the  granites  described  in  the  reports  of  the  40th  Parallel  Sur- 
vey into  three  groups  :  (1)  Those  of  Jurassic  age  ;  (2)  those  of 
Palaeozoic  age,  and  (3)  those  of  Archaean  age.  The  granites 
of  the  eastern  United  States,  on  the  other  hand,  have,  in  times 
past,  been  regarded  as  mainly  Archaean,  though  Dr.  Wadsworth 
has  shown  that  the  Quincy,  Massachusetts,  stone  is  an  eruptive 
rock  of  late  Primordial  or  more  recent  age,  while  Professor 
Hitchcock  regards  the  eruptive  granites  of  Vermont  as  having 
been  protruded  during  Silurian  or  perhaps  Devonian  times. 


THE  QUARTZ  PORPHYRIES  69 

(2)  THE  QUARTZ  PORPHYRIES 

Composition.  —  The  mineral  and  chemical  composition  of  the 
quartz  porphyries  is  essentially  the  same  as  that  of  the  gran- 
ites, from  which  they  differ  mainly  in  structure.  Their  essen- 
tial constituents  are  quartz  and  feldspar,  with  accessory  black 
mica  or  hornblende  in  very  small  quantities ;  other  acces- 
sories present,  as  a  rule  only  in  microscopic  quantities,  are 
magnetite,  pyrite,  hematite,  and  epidote. 

Structure.  —  The  prevailing  structure  is  porphyritic.  (Fig.  1, 
PI.  2.)  To  the  unaided  eye  they  present  a  very  dense  and  com- 
pact ground-mass  of  uniform  reddish,  brown,  black,  gray,  or  yel- 
lowish color,  through  which  are  scattered  clear  glassy  crystals 
of  quartz  alone,  or  of  quartz  and  feldspar  together.  The  quartz 
differs  from  the  quartz  of  granites  in  that  here  it  was  the  first 
mineral  to  separate  out  on  cooling,  and  hence  has  taken  on  a 
more  perfect  crystalline  form ;  the  crystal  outlines  of  the  feld- 
spar are  also  well  defined.  Under  the  microscope  the  ground- 
mass  in  the  typical  porphyry  is  found  to  consist  of  a  dense 
felsitic,  almost  irresolvable  substance,  which  chemical  analysis 
shows  to  be  also  a  mixture  of  quartzose  and  feldspathic  ma- 
terial. The  porphyritic  quartzes  show  frequently  a  marked 
corrosive  action  from  the  molten  magma,  the  mineral  having 
again  been  partially  dissolved  after  its  first  crystallization. 
(Fig:  3,  PI.  5.)  This  difference  in  structure  in  rocks  of  the 
same  chemical  composition  is  believed  to  be  due  wholly  to  the 
different  circumstances  under  which  the  two  rocks  have  solidi- 
fied from  a  molten  magma.  The  structure  of  the  ground-mass 
is  not  always  felsitic,  but  may  vary  from  a  glass,  as  in  the 
pitchstones  of  Meissen,  Isle  of  Arran,  and  the  Lake  Lugano 
region,  through  spherulitic,  micropegmatitic,  and  porphyritic 
to  perfectly  microcrystalline  forms  as  in  the  microgranites. 
This  difference  in  structure  may  be  best  understood  by  refer- 
ence to  Plate  5,  which  shows  the  microscopic  structure  of  (1) 
granite  from  Sullivan,  Hancock  County,  Maine,  (2)  micropeg- 
matite  from  Mount  Desert,  Maine,  and  (3)  a  quartz  porphyry 
from  Fairfield,  Pennsylvania.  Marked  fluidal  structure  is 
common.  (See  PI.  2,  Fig.  2.) 

Colors.  —  The  colors  of  the  ground-mass,  as  above  noted,  vary 
through  reddish,  brownish  gray  to  black  and  sometimes  yellowish 
or  green.  The  porphyritic  feldspars  vary  from  red,  pink,  and 


70  ROCKS  FORMED   THROUGH   IGNEOUS   AGENCIES 

yellow  to  snow-white,  and  often  present  a  beautiful  contrast  with 
the  ground-mass,  forming  a  desirable  stone  for  ornamental  pur- 
poses. 

Classification  and  Nomenclature. — Owing  to  the  very  slight 
development  of  the  accessory  minerals,  mica,  hornblende,  etc., 
it  has  been  found  impossible  to  adopt  the  system  of  classifica- 
tion and  nomenclature  used  with  the  granites  and  other  rocks. 
Vogelsang's  classification  as  modified  by  Rosenbusch  is  based 
upon  the  structure  of  the  ground-mass  as  revealed  by  the  micro- 
scope. It  is  as  follows:  — 

Ground-mass  holocrystalline  granular Micro-granite. 

Ground-mass  holocrystalline,  but  formed  of  quartz   and  feld- 
spar aggregates,  rather  than  district  crystals Granophyr. 

Ground-mass  felsitic Felsophyr. 

Ground-mass  glassy Vitrophyr. 

Intermediate  forms  are  designated  by  a  combination  of  the 
names,  as  granofelsophyr,  felsovitrophyr,  etc.  The  name  felsite 
is  often  given  to  rocks  of  this  group  in  which  the  porphyritic 
constituents  are  wholly  lacking.  The  names  felstone  arid 
petrosilex  are  also  common,  though  gradually  going  out  of  use. 
Elvanite  is  a  Cornish  miner's  term  and  too  indefinite  to  be  of 
great  value.  Eurite,  now  little  used,  applies  to  felsitic  forms. 
The  n&mQ  felsite  pitchstone  or  retinite  has  been  given  to  a  glassy 
form  with  pitch-like  lustre,  such  as  occurs  in  dikes  cutting  the 
old  red  sandstone  on  the  Isle  of  Arran.  Kugel  porphyry  is  a 
name  given  by  German  writers  to  varieties  showing  spheroids 
with  a  radiating  or  concentric  structure.  Micropegmatite  is  the 
term  not  infrequently  applied  to  such  as  show  under  the  micro- 
scope a  pegmatitic  structure.  (Fig.  2,  PI.  5.)  Various  popular 
names,  as  leopardite  and  toadstone,  are  sometimes  applied  to  such 
as  show  a  spotted  or  spherulitic  structure. 

(3)    THE   LIPARITES 

Mineral  Composition.  —  These  rocks  may  be  regarded  as  the 
younger  equivalents  of  the  quartz  porphyries,  or  the  volcanic 
equivalents  of  the  granites,  having  essentially  the  same  mineral 
and  chemical  composition.  The  prevailing  feldspar  is  the  clear 
glassy  variety  of  orthoclase  known  as  sanidin  ;  quartz  occurs  in 
quite  perfect  crystal  forms  often  more  or  less  corroded  by  the 
molten  magmas,  as  in  the  porphyries,  and  in  the  minute,  six- 


PLATE   7 


FIG.  1.   Liparite,  nevadite  form. 
FIG.  2.   Liparite,  rhyolite  form. 


FIG.  3.   Liparite,  obsidian  form. 
FIG.  4.   Liparite,  pumiceous  form. 


THE   LIPARITES 


71 


sided,  thin  platy  forms  known  as  tridymite.  The  accessory 
minerals  are  the  same  as  those  of  the  granites  and  quartz 
porphyries. 

Chemical  Composition.  —  Below  is  given  the  composition  of  : 
(I)  Nevadite,  from  the  northeastern  part  of  Chalk  Mountain, 
Colorado,  as  given  by  Cross.1  (II)  That  of  a  rhyolitic  form, 
from  the  Montezuma  Range,  Nevada,  as  given  by  King,2  and 
(III)  that  of  a  black  obsidian  from  the  Yellowstone  National 
Park,  Wyoming,  as  given  by  Iddings.3 


CONSTITUENTS 

I 

II 

III 

Silica  (Si02)  ...         

74.50% 

74.62% 

74  70  °L 

Alumina  (AlgOs)     

14.72 

11.96 

13.72 

Ferric  oxide  (Fe^Og)  

None 

1.20 

1.01 

Ferrous  oxide  (FeO)  
Ferric  sulphide  (FeS2) 

0.56 

0.10 

0.62 
0.40 

Manganese  (MnO) 

0.28 

Trace 

Lime  (CaO)    ... 

0.83 

0.36 

0.78 

Magnesia  (MgO)     .              .         . 

0.37 

0.14 

Soda  (Na2O)  

3.97 

2.26 

3.90 

Potash  (K20)     

4.53 

7.76 

4.02 

Phosphoric  anhydride  (P205)  

0.01 

Ignition. 

0.66 

1.02 

0.62 

100.38  % 

99.28  % 
2  2 

99.91  % 
2  3447 

Colors.  —  These  are  fully  as  variable  as  in  the  quartz  por- 
phyries ;  white,  though  all  shades  of  gray,  green,  brown,  yel- 
low, pink,  and  red  are  common.  Black  is  the  more  common 
color  for  the  glassy  varieties  of  obsidian,  though  they  are  often 
beautifully  spotted  and  streaked  with  red  or  reddish-brown. 

Structure.  — The  liparites  present  a  great  variety  of  structural 
features,  varying  from  holocrystalline,  through  porphyritic  and 
felsitic,  to  clear,  glassy  forms.  These  varieties  can  be  best 
understood  by  reference  to  Plates  5  and  7,  prepared  from 
photographs.  Fig.  1,  PI.  7,  is  that  of  the  coarsely  crystalline 
variety  nevadite  from  Chalk  Mountain,  Colorado  ;  Fig.  2  is 

1  Geology  and  Mining  Industry  of  Leadville,  Monograph  XII,  U.  S.  Geol. 
Survey,  p.  349. 

2  Geological  Exploration  40th  Parallel,  Vol.  I,  p.  652. 

3  Ann.  Rep.  U.  S.  Geol.  Survey,  1885-86,  p.  282. 


72  KOCKS  FORMED   THROUGH  IGNEOUS   AGENCIES 

that  of  a  common  felsitic  and  porphyritic  type ;  Fig.  3  is  that 
of  the  clear,  glassy  form,  obsidian  ;  Fig.  4  shows  also  an  obsid- 
ian, but  with  a  pumiceous  structure ;  Fig.  1  on  PI.  6  shows  the 
hollow  spherulites  or  lithopkyzce,  which  have  been  studied  and 
described  by  Professor  J.  P.  Iddings,  of  the  United  States  Geo- 
logical Survey.1  Such  forms  are  regarded  by  Mr.  Iddings  as 
.resulting  ufrom  the  action  of  absorbed  vapors  upon  the  molten 
glass  from  which  they  were  liberated  during  the  process  of 
crystallization  consequent  upon  cooling."  A  pronounced  flow 
structure  is  quite  characteristic  of  the  rocks  of  this  group,  as 
indicated  by  the  name  rhyolite.  The  microscopic  structure  of 
an  obsidian  is  shown  in  Fig.  4,  PI.  5.  Transitions  from  com- 
pact obsidian  into  pumiceous  forms,  due  to  expansion  of  included 
moisture,  are  common. 

Classification  and  Nomenclature.  —  The  following  varieties  are 
now  generally  recognized,  the  distinctions  being  based  mainly 
on  structural  features,  as  with  the  quartz  porphyries.  We  thus 
have  the  granitic-appearing  variety  nevadite,  the  less  markedly 
granular  and  porphyritic  variety  rhyolite,  and  the  glassy  forms 
hyaloliparite,  hyaline  rhyolite,  or  obsidian  as  it  is  variously  called. 
Hydrous  varieties  of  the  glassy  rock  with  a  dull  pitch-like  lustre 
are  sometimes  called  rhyolite  pitchstone. 

The  name  rhyolite,  from  the  Greek  word  pew,  to  flow,  it  may 
be  stated,  was  applied  by  Bichtofen  as  early  as  1860  to  this 
class  of  rocks  as  occurring  on  the  southern  slopes  of  the  Carpa- 
thians. Subsequently  Roth  applied  the  name  Liparite  to  similar 
rocks  occurring  on  the  Lipari  Islands.  The  first  name,  owing 
to  its  priority,  is  the  more  generally  used  for  the  group,  though 
Professor  Rosenbusch  in  his  latest  work  has  adopted  the  latter. 
The  name  Nevadite  is  from  the  state  of  Nevada,  and  was  also 
proposed  by  Richtofen.  The  name  Obsidian  as  applied  to  the 
glassy  variety  is  stated  to  have  been  given  in  honor  of  Obsid- 
ius,  its  discoverer,  who  brought  fragments  of  the  rock  from 
Ethiopia  to  Rome.  The  name  pantellerite  has  been  given  by 
Rosenbusch  to  a  liparite  in  which  the  porphyritic  constituent 
is  an  orthoclase. 

Rocks  of  these  types  occur,  in  the  United  States,  only  in 
the  regions  west  of  the  front  range  of  the  Rocky  Mountains. 
Apo-rhyolite  is  the  name  proposed  by  Dr.  Williams  for  the 

i  Obsidian  Cliff,  Yellowstone  National  Park,  Ann.  Rep.  U.  S.  Geol.  Survey, 
1885-86. 


THE    SYENITE-TRACHYTE    GROUP  73 

devitrified  and  otherwise  altered  pre-Cambrian  rhyolite  found 
at  South  Mountain  in  Pennsylvania. 

2.     THE   SYENITE-TRACHYTE  GROUP 

This  group  stands  next  to  that  of  the  granites  in  point  of 
acidity,  from  which  it  differs  mainly  in  the  lack  of  free  silica 
(quartz)  as  an  essential  constituent.  On  chemical  grounds  this 
and  the  next  group  to  be  described  belong  to  the  intermediate 
series,  standing  midway  between  the  acid  granites  and  the  basic 
basalts.  As  with  the  last,  we  have  plu tonic  and  effusive  forms. 
These  may  be  described  as  below  :  — 

(1)    THE    SYENITES 

The  name  Syenite,  from  Syene,  a  town  of  Egypt.  The  word 
was  first  used  by  Pliny  to  designate  the  coarse  red  granite  from 
quarries  at  Syene,  and  used  by  the  Egyptians  in  their  obelisks 
and  pyramids.  Afterwards  (in  1787)  Werner  introduced  the 
word  into  geological  nomenclature  to  designate  a  class  of  gran- 
ular rocks  consisting  of  feldspar  and  hornblende,  either  with  or 
without  quartz.  Later,  when  a  more  precise  classification 
became  necessary,  the  German  geologists  reserved  the  name 
syenite  to  designate  only  the  quartzless  varieties  of  these 
rocks,  while  the  quartz-bearing  varieties  were  referred  to  the 
hornblendic  granites.  This  is  the  classification  now  followed 
by  all  tlie  leading  petrologists  and  is  therefore  adopted  here. 
Much  confusion  has  arisen  from  the  fact  that  the  French  geolo- 
gist Roziere  insisted  upon  designating  the  quartz-bearing  rock 
as  syenite,  a  practice  which  has  been  followed  to  a  considerable 
extent  both  in  this  country  and  England. 

Mineral  Composition.  —  The  syenites  differ  from  the  granites 
only  in  the  absence  of  the  mineral  quartz,  consisting  essentially 
of  orthoclase  feldspar  in  company  with  biotite,  or  one  or  more 
minerals  of  the  amphibole  or  pyroxene  group.  A  soda-lime 
feldspar  is  nearly  always  present  and  frequently  microcline ; 
other  common  accessories  are  apatite,  zircon,  and  the  iron  ores : 
more  rarely  sodalite. 

Chemical  Composition.  —  In  column  I  below  is  given  the  com- 
position of  a  hornblende  syenite  from  near  Dresden,  Saxony, 
in  II  that  of  a  mica  syenite  (minette)  from  the  Odenwald,  and 
in  III  and  IV  that  of  augite-sodalite  syenites  from  Montana. 


KOCKS   FORMED   THROUGH  IGNEOUS   AGENCIES 


CONSTITUENTS 

I 

II 

ill 

IV 

Silica  (Si02)             .     .         .... 

60.02% 

57.37% 

54.15% 

56.45% 

Alumina  (AloOa)                                 . 

16.66 

13.84 

18.92 

20.08 

Ferric  iron  (F620g)                    .     .     . 

1 

f   2.44 

) 

f   1.31 

Ferrous  iron  (FeO)      

1   7.21 

t   3.44 

|   6.79 

I   4.39 

Magnesia  (MgO)     

2.51 

6.05 

1.90 

0.63 

Lime  (CaO)    

3.59 

5.53 

3.72 

2.14 

Soda  (Na20)                                    .    . 

2.41 

1.53 

5.47 

5.61 

Potash  (K20)           .              .... 

6.50 

4.47 

8.44 

7.13 

Ignition  (H2O)    

1.10 

3.17 

1.77 

Chlorine  (Cl  ) 

0.42 

0.43 

Phosphoric  acid  (PsO*-)        •              . 

0.13 

100.00  % 

97.84% 

99.81 

100.07  % 

Structure.  —  The  structure  of  the  syenites  is  wholly  analo- 
gous to  that  of  the  granites,  and  need  not  be  further  described 
here.  In  process  of  crystallization  the  apatite,  zircon,  and  iron 
ores  were  the  first  to  separate  out  from  the  molten  magma,  and 
hence  are  found  in  more  or  less  perfect  forms  enclosed  by  the 
feldspars  and  later-formed  minerals.  These  were  followed  in 
order  by  the  mica,  hornblende,  or  augite,  and  lastly  the  feld- 
spars, the  soda-lime  feldspars,  when  such  occur,  forming  subse- 
quent to  the  orthoclase. 

Color.  —  The  prevailing  colors  are  various  shades  of  gray, 
through  pink  to  reddish. 

Classification  and  Nomenclature.  —  According  as  one  or  the 
other  of  the  accessory  minerals  of  the  bisilicate  group  predomi- 
nates we  have  (1)  hornblende  syenite,  (2)  mica  syenite,  or  minette, 
and  (3)  augite  syenite. 

Other  varietal  names  have  from  time  to  time  been  given 
by  various  authors.  The  name  minette,  first  introduced  into 
geological  nomenclature  by  Voltz  in  1828  (Teall),  is  applied 
to  a  fine-grained  mica  orthoclase  rock,  occurring  only  in  the 
form  of  dikes  and  further  differing  from  the  typical  syenites  in 
having  a  porphyritic  rather  than  granitic  structure.  Vogesite 
is  the  name  applied  to  a  similar  rock  in  which  hornblende  or 
augite  prevails  in  place  of  mica.  These  rocks  are  placed  by 
Professor  Rosenbusch  in  his  latest  work  in  the  group  of 
syenitic  lamprophyrs.  Monzonite  is  a  varietal  name  for  the 
augite  syenite  of  Monzoni  in  the  Tyrol. 


THE  ORTHOCLASE  OR  QUARTZ-FREE  PORPHYRIES     75 

The  mode  of  occurrence  of  the  syenites  is  similar  to  that 
of  the  granites,  though  they  are  much  more  limited  in  their 
distribution.  In  the  United  States  they  have  thus  far  been 
described  but  sparingly.  Marbleheacl  Neck,  Massachusetts ; 
Jackson,  New  Hampshire,  are  well-known  localities ;  a  beauti- 
ful hornblende  syenite  is  found  among  the  glacial  drift  boulders 
about  Portland,  Maine,  but  its  exact  source  is  not  known.  The 
hornblende  syenite  described  by  Hawes  as  occurring  at  Red 
Hill,  Moultonborough,  New  Hampshire,  has  been  shown  by 
Professor  W.  S.  Bayley1  to  carry  elseolite,  and  to  belong  to  the 
group  of  elseolite  syenites.  Hornblende  syenites  occur  in  the 
Vosgea  Mountains  of  Germany  and  in  Saxony ;  mica  syenites 
or  minettes  in  the  Odenwald,  Germany,  Baden,  Saxony,  and  in 
the  Fichtelgebirge.  A  mica-augite  syenite  carrying  sodalite 
occurs  as  a  Cretaceous  eruptive  in  Jefferson  County,  Montana,2 
and  a  similar  rock  has  been  described  by  Lindgreii  from  the 
Highwood  Mountains  in  the  saine  state.3 

(2)    THE   ORTHOCLASE    OR    QUARTZ-FREE   PORPHYRIES 

Mineral  Composition.  —  The  essential  constituents  are  the 
same  as  those  of  syenite.  They  consist  therefore  of  a  compact 
porphyry  ground-mass  with  porphyritic  feldspar  (orthoclase) 
and  accessory  plagioclase,  quartz,  mica,  hornblende,  or  minerals 
of  the  pyroxene  group.  More  rarely  occur  zircon,  apatite, 
magnetite,  etc.,  as  in  the  syenites. 

Chemical  Composition. —  Being  poor  in  quartz,  these  rocks  are 
a  trifle  more  basic  than  the  quartz  porphyries  which  they  other- 
wise resemble.  The  following  is  the  composition  of  an  ortho- 
clase porphyry  from  Predazzo  as  given  by  Kalkowski : 4  Silica, 
64.45%  ;  alumina,  16.31%  ;  ferrous  oxide,  6.49%  ;  magnesia, 
0.30%;  lime,  1.10%;  soda,  5.00%;  potash,  5.45%;  water, 
0.85  %. 

Structure.  —  Excepting  that  orthoclase  is  the  porphyritic  con- 
stituent, they  are  structurally  identical  with  the  quartz  porphy- 
ries, and  need  not  be  further  described  here. 

Colors.  --These  are  the  same  as  the  quartz  porphyries  already 
described. 

1  Bull.  Geol.  Soc.  of  America,  Vol.  IK,  1802. 

2  Proc.  U.  S.  Nat.  Museum,  Vol.  XVII,  1894. 

3  Proc.  Cali.  Acad.  of  Sciences,  Vol.  Ill,  2d  series,  p.  47. 

4  Elemente  der  Lithologie,  p.  80. 


76  ROCKS   FORMED   THROUGH   IGNEOUS   AGENCIES 

Classification  and  Nomenclature.  —  The  orthoclase  or  quartz- 
free  porphyries  bear  the  same  relation  to  the  syenites  as  do  the 
quartz  porphyries  to  granite,  and  the  rocks  are  frequently 
designated  as  syenite  porphyries.  Like  the  quartz  porphyries, 
they  occur  in  intrusive  sheets,  dikes,  and  lava  flows  associated 
with  the  Palaeozoic  formations.  Owing  to  the  frequent  absence 
of  accessory  minerals  of  the  ferro-magnesia  group,  the  rocks  can- 
not in  all  cases  be  classified  as  are  the  syenites,  and  distinctive 
names  based  upon  other  features  are  often  applied.  The  term 
orthopTiyr  is  applied  to  the  normal  orthoclase  porphyries,  and 
these  are  subdivided  when  possible  into  biotite,  hornblende,  or 
augite  orthophyr  according  as  either  one  of  these  minerals  is  the 
predominating  accessory.  The  term  rhombporphyry  has  been 
used  to  designate  an  orthoclase  porphyry  found  in  southern 
Norway,  and  in  which  the  porphyritic  constituent  appears  in 
characteristic  rhombic  outlines,  and  which  is  further  distin- 
guished by  a  complete  absencre  of  quartz  and  rarity  of  horn- 
blende. The  name  keratophyr  has  been  given  by  Gumbel  to  a 
quartzose  or  quartz-free  porphyry  containing  a  sodium-rich  alka- 
line feldspar.  So  far  as  can  be  at  present  judged,  rocks  of  this 
type  are  much  more  restricted  in  their  occurrence  than  are  the 
quartz  porphyries  already  described. 


(3)    THE   TRACHYTES 

Trachyte,  from  the  Greek  word  rpa^v^  rough,  in  allusion  to 
the  characteristic  roughness  of  the  rock.  The  term  was  first 
used  by  Hauy  to  designate  the  well-known  volcanic  rocks  of  the 
Drachenfels  on  the  Rhine. 

Mineral  Composition.  —  Under  the  name  of  trachyte  are  com- 
prehended those  massive  Tertiary  and  post-Tertiary  lavas,  con- 
sisting essentially  of  sanidin  with  hornblende  augite  or  black 
mica,  and  which  may  be  regarded  as  the  younger  equivalents  of 
of  the  quartz-free  porphyries.  The  common  accessory  minerals 
are  plagioclase,  tridymite,  apatite,  spheiie,  and  magnetite,  more 
rarely  olivine,  sodalite,  humite,  hauyne,  and  melilite. 

Chemical  Composition.  —  The  following  analyses  show  the 
range  in  chemical  composition  of  these  rocks,  I  being  that  of 
the  trachyte  of  Game  Ridge,  Colorado,  and  II  that  of  a  La 
Guardia  stone. 


THE   FOYAITE-PHONOLITE   GROUP 


77 


CONSTITUENTS 

I 

II 

Silica  (SiOo)     

66  03  °/ 

fy»  00  <V 

Alumina  (A12O3)      

18  49 

oo.ujy0 

96  OQ 

Ferric  oxide  (Fe203)     

2  18 

Manganese  oxide  (MnO)  

Trace 

Tnrp 

Lime  (CaO)      

0  96 

q  41 

Magnesia  (MgO)  

0  39 

9  70 

Potash  (K20)  

5  86 

6  4Q 

Soda  (Na20)    

5  22 

Q    QQ 

Ignition  (H20) 

0  85 

1  0^ 

Phosphoric  acid  (PoOs) 

0  04 

Total     

100  24  °/ 

100.74  <y 

Structure.  —  In  structure  the  trachytes  are  rarely  granular, 
but  possess  a  fine,  scaly  or  microfelsitic  ground-mass,  rendered 
porphyritic  through  the  development  of  scattering  crystals  of 
sanidin,  hornblende,  augite  of  black  mica.  The  texture  is 
porous,  and  the  rock  possesses  a  characteristic  roughness  to 
the  touch;  hence  the  derivation  of  the  name  as  given  above. 
Perlitic  structure  is  common  in  the  glassy  forms.  The  micro- 
scopic structure  of  the  trachyte  of  Monte  Vetta  is  shown  in 
Fig.  5,  PI.  5. 

Colors.  —  The  prevailing  colors  are  grayish,  yellowish,  or 
reddish. 

Classification  and  Nomenclature.  —  They  are  divided  into  horn- 
blende, auyite,  or  mica  trachytes,  according  as  any  one  of  these 
minerals  predominates.  The  name  sanidin-oligoclase  trachyte  is 
sometimes  given  to  trachytes  in  which  both  these  feldspars  ap- 
pear as  prominent  constituents.  The  presence  of  quartz  gives 
rise  to  the  variety  quartz  trachytes.  (See  .under  rhyolite.)  The 
glassy  form  of  trachyte  is  commonly  known  under  the  name  of 
trachyte  pitchstone,  or  if  with  a  perlitic  structure  simply  as  per- 
lite.  In  his  most  recent  work  Professor  Rosenbusch  has  included 
the  glassy  forms  under  the  name  of  hyalotrachyte. 

3.     THE  FOYAITE-PHONOLITE   GROUP 

This  group  differs  from  the  last  mainly  in  the  partial  replace- 
ment of  the  potash  feldspars  by  the  closely  related  mineral 
ekeolite  or  nepheline.  It  includes  therefore  those  plutonic  and 
effusive  rocks  commonly  known  under  the  name  of  elceolite  or 


78 


ROCKS   FORMED  THROUGH  IGNEOUS   AGENCIES 


nepheline  syenites  and  the  phonolites.  In  their  silica  and  potash 
percentages  it  will  be  observed  they  differ  not  greatly  from  the 
syenites  proper,  but  are  much  more  rich  in  soda  and  corre- 
spondingly poor  in  lime.  They  may  be  described  in  detail  as 
follows  :  — 

(1)    THE   NEPHELINE    (ELJEOLITE)    SYENITES:     FOYAITS 


Nepheline  from  the  Greek  i/e^eX?;,  a  cloud,  since  the  mineral 
becomes  cloudy  on  immersion  in  acid.  Elaeolite  from  e\alov,  oil, 
in  allusion  to  the  greasy  lustre.  Syenite  from  Syene  in  Egypt. 

Mineral  Composition.  —  The  essential  constituents  of  this 
group  are  nepheline  (elyeolite)  and  orthoclase,  with  nearly 
always  a  pyroxenic  or  amphibolic  mineral  and  a  plagioclase 
feldspar.  The  common  accessory  minerals  are  sphene,  sodalite, 
cancrinite,  zircon,  apatite,  black  mica,  and  the  iron  ores  ilme- 
nite  and  magnetite,  with  occasional  leucite,  eucolite,  melino- 
phane,  and  also  tourmalines,  perowskite,  and  olivine.  Calcite, 
epidote,  chlorite,  analcite,  and  sundry  minerals  of  the  zeolite 
group  occur  as  secondary  products. 

Professor  W.  S.  Bayley  has  computed1  the  relative  propor- 
tions of  the  various  constituents  in  the  elseolite  syenite  of  Litch- 
field,  Maine,  as  follows  :  Elaeolite,  17  %  ;  potash  feldspar,  27  %  ; 
albite,  47%  ;  cancrinite,  2  %  ;  and  black  mica  (lepidomelane),  7%. 

Chemical  Composition.  —  The  composition  of  the  elseolite  sye- 
nite from  several  well-known  localities  is  given  below  :  — 


CONSTITUENTS 

ALGKAVE, 
PORTUGAL 

HOT  SPRINGS, 
ARKANSAS 

LlTCHFIELD, 

MAINE 

BEEMERVILLE, 
NEW  JERSEY 

Silica  (Si02) 

54    61   % 

59.70% 

60.39% 

50.36% 

Alumina  (A1203)       .     . 
Ferric  oxide  (Fe20a)    .     .     . 
Ferrous  oxide  (FeO)     .     .     . 
Magnesia  (MgO)  

22.07 
2.33 
2.50 
0  88 

18.85 
4.85 

0  68 

22.51 
.42 
2.26 
0.13 

19.84 
J6.94 

Manganese  oxide  (MnO)  .  . 
Lime  (CaO) 

2  51 

1  34 

0.08 
0  32 

0.411 
343 

Soda  (Na2O)  

7  58 

6  29 

8  44 

7.64 

Potash  (K2O)  

5  46 

5  97 

4.77 

7.17 

Titanium  oxide  (Ti02)  .  . 
Phosphoric  anhydride  (P205) 
Water  (H2O)  

0.09 
0.15 
1  13 

1  88 

.57 

3.512  (loss) 

1  Bull.  Geol.  Soc.  of  America,  Vol.  Ill,  1892,  p.  231. 


THE   NEPHELINE   SYENITES  79 

The  essential  points  to  be  noted  are  the  larger  percentages 
of  the  alkalies  over  those  yielded  by  syenites  of  the  ordinary 
type,  or  the  granites. 

Color.  —  The  colors  are  light  to  dark  gray,  and  sometimes 
reddish. 

Structure.  —  The  syenites,  like  the  granites,  are  massive  holo- 
crystalline  granular  rocks,  and  as  a  rule  sufficiently  coarse  in 
texture  to  allow  a  partial  determination  of  the  constituent 
parts  by  the  unaided  eye.  In  the  Litchfield  (Maine)  syenite 
the  elgeolite  often  occurs  in  crystals  upwards  of  5  centimetres 
in  length,  and  zircons  2  centimetres  in  length  are  not  rare. 
Neither  of  the  essential  constituents  occur  in  the  form  of  per- 
fect crystals,  while  the  apatite,  zircon,  black  mica,  and  pyrox- 
enic  constituents  often  present  very  perfect  forms.  The  can- 
crinite  occurs  both  as  secondary  after  the  elieolite  and  as  a 
primary  constituent  in  the  form  of  long  needle-like  yellow 
crystals  with  a  hexagonal  outline.  This  last  form  is  especially 
characteristic  of  the  Litchfield  rock.  The  sodalite  occurs  both 
as  crystals  and  in  irregular  massive  forms,  coating  the  walls  of 
crevices. 

Classification  and  Nomenclature.  —  Several  varietal  names  have 
been  given  to  the  rocks  of  this  group  as  described  by  various 
authors.  Miascite  was  the  name  given  by  G.  Rose  to  the  sye- 
nite occurring  at  Miask  in  the  Urals  ;  Ditroite  to  that  occurring 
at  Ditro  in  Transylvania,  and  Foyaite,  by  Blum,  to  that  from 
Mount  Foya,  in  the  province  of  Algrave  in  Portugal.  The 
name  zircon  syenite,  or  Laurvikite,  has  been  given  to  the  vari- 
ety from  Laurvig  in  southern  Norway,  which  is  rich  in  zircons. 
Tinguaite  is  the  name  proposed  for  a  varietal  form  from  Serra 
de  Tingua,  province  of  Rio  Janeiro,  Brazil. 

American  petrographers  have  not  been  at  all  delinquent  in 
the  matter  of  names,  and  have  added  to  an  already  over- 
burdened nomenclature  such  terms  as  Litchfieldite,  Ouachitite, 
Pulaskite,  and  Fourchite  to  varieties  from  Litchfield  (Maine) 
and  the  Hot  Springs  region  of  Arkansas.  Liebnerite  is  the 
name  given  to  an  eleeolite  syenite  porphyry  occurring  in  the 
Tyrol. 

Rocks  of  this  group,  although  wide-spread  in  their  distribu- 
tion, are  nevertheless  not  abundant.  The  more  important 
localities  thus  far  described  have  already  been  noted  ;  there 
remains  to  be  mentioned  only  the  locality  at  Red  Hill,  Moul- 


80  ROCKS   FORMED   THROUGH  IGNEOUS  AGENCIES 

tonborough,  New  Hampshire,  the  rock  of  which  was  first  de- 
scribed as  an  ordinary  syenite,  and  that  of  Hastings  County, 
Ontario. 

(2)   THE   PHONOLITES 

Phonolite,  from  the  Greek  word  <f>a>vri,  sound,  and  \l0os,  stone, 
in  allusion  to  the  clear  ringing  or  clinking  sound  which  slabs 
of  the  stone  emit  when  struck  with  a  hammer ;  formerly  called 
clinkstone  for  the  same  reason. 

Mineral  Composition.  —  The  phonolites  consist  essentially  of 
sanidin  and  nepheline  or  leucite,  together  with  one  or  more 
minerals  of  the  augite-hornblende  group,  and  generally  hauyne 
or  nosean.  The  common  accessories  are  plagioclase,  apatite, 
sphene,  mica,  and  magnetite ;  more  rarely  occur  tridymite, 
melanite,  zircon,  and  olivine.  The  rock  undergoes  ready  alter- 
ation, and  calcite,  chlorite,  limonite,  and  various  minerals  of 
the  zeolite  group  occur  as  secondary  products. 

Chemical  Composition.  —  The  average  of  six  analyses  given 
by  Zirkel1  is  as  follows:  Silica,  58.02%;  alumina,  20.03%; 
iron  oxides,  6.18  %;  manganese  oxide,  0.58%;  lime,  1.89%; 
magnesia,  0.80  % ;  potash,  6.18  % ;  soda,  6.35  % ;  water,  1.88  % ; 
specific  gravity,  2.58. 

Structure.  —  The  phonolites  present  but  little  variety  in 
structure,  being  usually  porphyritic,  seldom  evenly  granular. 
The  porphyritic  structure  is  due  to  the  development  of  large 
crystals  of  sanidin,  nepheline,  leucite,  or  hauyne,  and  more  rarely 
hornblende,  augite,  or  sphene,  in  the  fine-grained  and  compact 
ground-mass,  which  is  usually  microcrystalline,  rarely  glassy  or 
amorphous. 

Colors.  —  The  prevailing  colors  are  dark  gray  or  greenish. 

Classification  and  Nomenclature.  —  Three  varieties  are  recog- 
nized by  Professor  Rosenbusch,  the  distinction  being  founded 
upon  the  variation  in  proportional  amounts  of  the  three  miner- 
als, sanidin,  nepheline,  or  leucite.  We  thus  have  (1)  nepheline 
phonolite,  consisting  essentially  of  nepheline  and  sanidin,  and 
which  may  therefore  be  regarded  as  the  volcanic  equivalent 
of  the  nepheline  syenite ;  (2)  leucite  phonolite,  consisting 
essentially  of  leucite  and  sanidin  ;  and  (3)  leucitophyr,  which 
consists  essentially  of  both  nepheline  and  leucite  in  connection 
with  sanidin,  and  nearly  always  melanite. 

1  Lehrbuch  der  Petrographie,  II,  p.  193. 


THE   DIORITE-ANDESITE   GROUP  81 

So  far  as  now  known,  these  rocks  are  of  comparatively  rare 
occurrence  in  the  United  States,  having  been  described  as 
occurring  only  in  the  Black  Hills  of  South  Dakota  and  the 
Cripple  Creek  district  of  Colorado. 

4.     THE  DIORITE-ANDESITE  GROUP 

We  come  now  to  groups  of  rocks  which  show  a  still  greater 
falling  off  in  their  total  amount  of  silica,  as  indicated  by  analy- 
ses, and  a  like  diminution  in  the  amount  of  potash.  The  cause 
of  this  falling  off  is  due  to  the  absence  as  an  essential  constituent 
of  quartz  and  potash  feldspars,  the  latter  being  replaced  by  soda- 
lime  varieties,  and  which  in  their  turn  cause  a  corresponding  in- 
crease in  the  elements  sodium  and  calcium.  The  group  includes 
the  plutonic  type  diorite,  and  the  effusive  types  hornblende  por- 
phyrite,  and  andesite.  These  may  be  described  as  below:  — 

(1)    THE   DIORITES   (GREENSTONES  IN  PART) 

Diorite,  from  the  Greek  word  Siopi^elv,  to  distinguish.     Term 


first  used  by  the  mineralogist  Haiiy. 

Mineral  Composition.  —  The  essential  constituents  of  diorite 
are  plagioclase  feldspar,  either  labradorite  or  oligoclase,  and 
hornblende  or  black  mica.  The  common  accessories  are  mag- 
netite, titanic  iron,  orthoclase,  apatite,  epidote,  quartz,  augite, 
black  mica,  and  pyrite,  more  rarely  garnets.  Calcite  and 
chlorite  occur  as  alteration  products. 

Structure.  —  Dioritesare  holocrystalline  granular  rocks,  and  as 
a  rule,  massive,  though  schistose  forms  occur.  The  individual 
crystals  composing  the  rock  are  sometimes  grouped  in  globular 
aggregates,  thus  forming  the  so-called  orbicular  diorite,  kuyel 
diorite,  or  napoleonite  from  Corsica.  (Fig.  1,  PI.  8.)  The 
texture  is,  as  a  rule,  fine,  compact,  and  homogeneous,  and  its 
true  nature  discernible  only  with  the  aid  of  a  microscope  ;  more 
rarely  porphyritic  forms  occur  as  in  the  camptonites. 

Colors.  T—  The  colors  vary  from  green  and  dark  gray  to 
almost  black. 

Chemical  Composition.  —  The  following  table  shows  the  wide 
range  in  chemical  composition  found  in  rocks  commonly  grouped 
under  this  head. 

Classification.  —  Accordingly  as  they  vary  in  mineral  comno- 
sition  the  diorites  are  classified  as  (1)  diorite,  in  which  horn- 


82 


ROCKS   FORMED   THROUGH  IGNEOUS  AGENCIES 


CONSTITUENTS 

I 

II 

III 

IV 

V 

VI 

Silica  (Si02) 

67  54  % 

61.75% 

56.71  % 

50.47  % 

43.50% 

39  32  % 

Titanic  oxide  (TiO  )  . 

1.70 

Alumina  (A1203)  .  .  . 
Ferric  iron  (Fe203)  .  . 
Ferrous  iron  (FeO)  .  . 
Manganese  oxide  (MnO) 

17.02 
2.97 
0.04 

18.88 
0.52 
3.52 

18.36 
6.45 

18.73 
4.19 
4.92 

17.02 
13.68 

14.48 
2.01 

8.73 
0.71 

Lime  (CaO) 

2  94 

3  54 

6.11 

8  82 

8  15 

8.30 

Magnesia  (MgO)  .  .  . 
Potash  (K2O)  .... 
Soda  (Na20)  

1.51 

2.28 
4.62 

1.90 
1.24 
3.67 

3.92 
2.38 
3.52 

3.48 
3.56 
4.62 

6.84 
2.84 
2.84 

11.11 
0.87 
3.76 

Phosphoric  acid  (P2O5)  . 
Carbonic  acid  (CO2)  .  . 
Water  (H20)  .... 

1  0.55 

4.46 

0.58 

4.35 

0.61 
5.25 

2.57 

I.  Quartz-mica  diorite:  Electric  Peak,  Yellowstone  Park  (J.  P.  Iddings). 
II.  Diorite:  Pemnaen-Mawr,  Wales  (J.A.Phillips).  III.  Diorite:  Comstock 
Lode,  Nevada  (40th  Parallel  Survey).  IV.  Augite  diorite :  Custer  County, 
Colorado  (Whitman  Cross).  V.  Porphyritic  diorite  (cainptonite)  :  Fairhaven, 
Vermont  (J.  F.  Kemp).  VI.  Porphyritic  diorite:  Lewiston,  Maine  (G.  P. 
Merrill). 

blende  alone  is  the  predominating  accessory;  (2)  mica  diorite, 
in  which  black  mica  replaces  the  hornblende,  and  (3)  augite 
diorite,  in  which  the  hornblende  is  partially  replaced  by  augite. 
The  presence  of  quartz  gives  rise  to  the  varieties,  quartz,  quartz 
augiie,  and  quartz-mica  diorites.  The  name  tonalite  has  been 
given  by  Vom  Rath  to  a  quartz  diorite  containing  the  feldspar 
andesine  and  very  rich  in  black  mica.  Kersantite  is  a  dioritic 
rock  occurring,  so  far  as  known,  only  in  dikes,  and  consisting 
essentially  of  black  mica  and  plagioclase,  with  accessory  apatite 
and  augite,  or  more  rarely  hornblende,  quartz,  and  orthoclase. 
It  differs  from  the  true  mica  diorite  in  being,  as  a  rule,  of  a 
porphyritic  rather  than  granitic  structure.  Professor  Rosen- 
busch,  in  his  latest  work,  has  placed  the  kersantites,  together 
with  the  porphyritic  diorites  (camptonites),  under  the  head 
of  dioritic  lamprophyrs  in  the  class  of  dike  rocks  or  "gange- 
steine."  The  name,  it  should  be  stated,  is  from  Kersanton,  a 
small  hamlet  in  the  Brest  Roads,  department  of  Finistere, 
France. 

The  diorites  were  formerly,  before  their  exact  mineralogical 
nature  was  well  understood,  included  with  the  diabases  and 
melaphyrs  under  the  general  name  greenstone  (Ger.  G-riinsteiii). 


PLATE   8 


FIG.  1.  Orbicular  diorite. 


Fia.  2.   Granite  spheroid. 


THE   PORPHYRITES   AND   ANDESITES  83 

They  are  rocks  of  wide  geographic  distribution,  but  apparently 
less  abundant  in  the  United  States  than  are  the  diabases.  The 
lamprophyr  varieties  are  still  less  abundant,  so  far  as  now 
known. 

(2)    THE   PORPHYRITES 

Mineral  and  Chemical  Composition.  —  The  essential  constitu- 
ents of  the  porphyrites  are  the  same  as  of  the  diorites,  from 
which  they  differ  mainly  in  structure. 

Structure.  —  The  porphyrites,  as  a  rule,  show  a  felsitic  or 
glassy  ground-mass,  as  do  the  quartz  porphyries,  in  which  are 
embedded  quite  perfectly  developed  porphyritic  plagioclases, 
with  or  without  hornblende  or  black  mica.  At  times,  as  in 
the  well-known  "  porfido  rosso  antico,"  or  antique  porphyries 
of  Egypt,  the  ground-mass  is  microcrystalline,  forming  thus 
connecting  links  between  the  true  diorites  and  diorite  porphy- 
rites. Indeed,  the  rocks  of  the  group  may  be  said  to  bear  the 
same  relation  to  the  diorites  in  the  plagioclase  series  as  do 
the  quartz  porphyries  to  the  granites  in  the  orthoclase  series, 
or  better  yet,  they  may  be  compared  with  the  hornblende  an- 
desites,  of  which  they  are  apparently  the  Palaeozoic  equivalents. 

Colors.  —  The  prevailing  colors  are  dark  brown,  gray,  or 
greenish. 

Classification.  —  According  to  the  character  of  prevailing 
accessory  mineral,  we  have  hornblende  porphyrite,  or  diorite 
porphyrite,  as  it  is  sometimes  called,  and  mica  porphyrite. 
When,  as  is  frequently  the  case,  neither  of  the  above  minerals 
are  developed  in  recognizable  quantities,  the  rock  is  designated 
as  simply  porphyrite.  The  porphyrites  are  wide-spread  rocks, 
very  characteristic  of  the  later  Paleozoic  formations,  occurring 
as  contemporaneous  lava  flows,  intrusive  sheets,  dikes,  and 
bosses. 

(3)    THE   ANDESITES 

The  name  Andesite  was  first  used  by  L.  V.  Buch  in  1835,  to 
designate  a  type  of  volcanic  rocks  found  in  the  Andes  Moun- 
tains, South  America. 

Mineral  Composition.  —  The  essential  constituents  are  soda- 
lime  feldspar,  together  with  black  mica,  hornblende,  augite,  or 
a  rhombic  pyroxene,  and  in  smaller,  usually  microscopic  pro- 
portions, magnetite,  ilmenite,  hematite,  and  apatite.  Common 


84 


ROCKS  FORMED   THROUGH   IGNEOUS  AGENCIES 


accessories  are  olivine,  sphene,  garnets,  quartz,  tridymite,  anor- 
thite,  sanidin,  and  pyrite. 

Chemical  Composition.  —  The  composition  of  the  andesites 
varies  very  considerably,  the  quartz-bearing  members  naturally 
showing  much  the  higher  percentage  of  silica.  The  following 
table  shows  the  composition  of  a  few  typical  forms  :  — 


CONSTITUENTS 

I 

II 

III 

IV 

V 

VI 

Silica  (Si02) 

66.32  % 
14.33 
5.53 
0.25 
2.45 
4.64 
3.90 
1.61 
1.13 

69.51  % 
15.75 
3.34 

2.09 
1.71 
3.89 
3.34 

61.12% 
11.61 
11.64 

0.61 
4.33 
3.85 
3.52 
4.35 

56.07  % 
19.06 
5.39 
0.92 
2.12 
7.70 
4.52 
1.24 
0.99 

56.19% 
16.21 
4.92 
4.43 
4.60 
7.00 
2.96 
2.37 
1.03 

58.33% 
18.17 

6.03 
2.40 
6.19 
3.20 
3.02 
0.76 

Alumina  (A1203)     .     .     . 
Ferric  oxide  (Fe20s)  .     . 
Ferrous  oxide  (FeO)  .     . 
Magnesia  (MgO)     .     .     . 
Lime  (CaO) 

Soda  (Na«>0) 

Potash  (K20)     .... 
Water  (H20)      .... 

100.16% 

99.63% 

101.03% 

98.01  % 

99.62 

98.10% 

I.  Dacite  from  Kis  Sebes,  Transylvania.  II.  Dacite  from  Lassens  Peak, 
California.  III.  Hornblende  andesite  from  hill  north  of  Gold  Peak,  Nevada. 
IV.  Hornblende  andesite  from  Bogoslof  Island,  Alaska.  V.  Hypersthene  ande- 
site, Buffalo  Peaks,  Colorado.  VI.  Augite  andesite  from  north  of  American 
Flat,  Washoe,  Nevada. 

Structure.  —  To  the  unaided  eye  the  andesites  present  as  a 
rule  a  compact,  often  rough  and  porous  ground-mass  carrying 
porphyritic  feldspars  and  small  scales  of  mica,  hornblende,  or 
whatever  may  be  the  prevailing  accessory;  pumiceous  forms 
are  not  uncommon.  Under  the  microscope  the  ground-mass  is 
found  to  vary  from  clear  glassy  through  microlitic  forms  to 
almost  holocrystalline.  The  minerals  of  the  ground-mass  are 
feldspars  in  elongated  microlites,  specks  of  iron  ore,  apatite  in 
very  perfect  forms,  and  one  or  more  of  the  accessory  ferro-mag- 
nesian  minerals.- 

Colors.  —  The  prevailing  colors  are  some  shade  of  gray,  green- 
ish or  reddish. 

Classification  and  Nomenclature.  —  Specific  names  are  given 
dependent  upon  the  character  of  the  prevailing  accessory.  We 
thus  have :  — 

Andesites  with  quartz  =  Quartz  andesites  or  dacites. 

Andesites  in  which  hornblende  prevails  =  Hornblende  andesites. 


THE   GABBRO-BASALT   GROUP  85 

Andesites  in  which  augite  prevails  =  Augite  andesites. 
Andesites  in  which  hypersthene  prevails  =  Hypersthene  andesites. 
Andesites  in  which  mica  prevails  =  Mica  andesites. 

The  glassy  varieties  are  often  known  as  hyaline  andesites. 
The  name  propylite  was  given  by  Richthofen  to  a  group  of 
andesitic  rocks  prevalent  in  Hungary,  Transylvania,  and  the 
western  United  States,  but  these  rocks  have  since  been  shown 
by  Dr.  Wadsworth J  and  others  to  be  but  altered  andesites,  and 
the  name  has  fallen  largely  into  disuse. 

5.     THE  GABBRO-BASALT  GROUP 

We  have  here  a  large  and  variable  group  of  rocks  which  on 
structural  and  mineralogical  grounds  might  well  be  subdivided. 
Thus  the  gabbros,  norites,  and  hypersthene  andesites  might 
well  be  considered  as  a  group  by  themselves,  while  the  diabases, 
augite  porphyrites,  melaphyrs,  and  basalts  could  form  a  second. 
Owing,  however,  to  the  similarity  of  the  magmas  from  which 
they  have  been  derived,  it  is  believed  the  wants  of  the  student 
will  be  best  subserved  by  grouping  them  all  together  as  above. 
They  may  be  described  in  detail  as  below  :  — 

(1)    THE   GABBROS 

Gabbro,  an  old  Italian  name  originally  applied  to  serpen- 
tinous  rocks  containing  diallage. 

Mineral  Composition.  —  The  gabbros  consist  essentially  of  a 
basic  soda-lime  feldspar,  either  labradorite,  bytownite,  or  an- 
orthite,  and  diallage  or  a  closely  related  monoclinic  pyroxene, 
a  rhombic  pyroxene  (enstatite  or  hypersthene),  and  more  rarely 
olivine.  Apatite  and.  the  iron  ores  are  almost  universally  pres- 
ent, and  often  picotite,  chromite,  pyrrhotite,  more  rarely  com- 
mon pyrites,  and  a  green  spinel.  Secondary  brown  mica  and 
hornblende  are  common.  Quartz  occurs  but  rarely. 

Chemical  Composition.  —  As  with  other  groups,  the  percent- 
age amounts  of  the  various  constituents  obtained  by  analyses 
is  dependent  upon  the  relative  proportion  of  the  constituent 
minerals.  In  the  tables  given  below,  analyses  like  I  and  III, 
showing  very  little  iron  and  magnesia,  but  rich  in  lime  and 
soda  and  alumina,  are  of  rocks  in  which  the  pyroxenic  con- 

1  Proc.  Boston  Society  of  Natural  History,  Vol.  XXI,  1881,  p.  260. 


86 


KOCKS   FORMED   THROUGH   IGNEOUS  AGENCIES 


stituents  are  almost  wholly  lacking,  and  which  consist  essen- 
tially of  lime  feldspars  only. 


CONSTITUENTS 

I 

II 

Ill 

IV 

V 

VI 

Silica  (Si02)  

59.55  % 

54.72  % 

53.43  % 

49.15% 

46.85% 

45.66% 

Alumina  (A12O3)    .     .     . 

25.62 

17.79 

28.01 

21.90 

19.72 

16.44 

Ferric  iron  (Fe203)     .     . 

0.75 

2.08 

0.75 

6.60 

3.22 

0.66 

Ferrous  iron  (FeO)     . 

.... 

6.03 

.... 

4.54 

7.99 

13.90 

Lime  (CaO)  

7.73 

6.84 

11.24 

8.22 

13.10 

7.23 

Magnesia  (MgO)     . 

Trace 

5.85 

0.63 

3.03 

7.75 

11.57 

Potash  (K20)     .... 

0.96 

3.01 

0.96 

1.61 

0.09 

0.41 

Soda  (Na2O)  

5.09 

3.02 

4.85 

3.83 

1.56 

2.13 

Ignition  and  loss     .     .     . 

0.45 

.... 

.... 

1.92 

0.56 

0.07 

I.  Anorthosite  :  Chateau  Richer,  Canada  (T.  S.  Hunt).  II.  Gabbro  :  near 
Cornell  Dam,  Croton  River,  New  York  (J.  F.  Kemp).  III.  Anorthosite: 
Labrador  (A.  Wickman).  IV.  Gabbro :  near  Duluth,  Minnesota  (Streng). 
V.  Gabbro:  near  Baltimore,  Maryland  (G.  H.  Williams).  VI.  Gabbro:  North- 
west Minnesota  (W.  S.  Bay  ley). 

Structure.  —  The  gabbro  structure  is  quite  variable.  Like 
the  other  plutonic  rocks  mentioned,  they  are  crystalline  granu- 
lar, the  essential  constituents  rarely  showing  perfect  crystal 
outlines.  As  a  rule  the  pyroxenic  constituent  occurs  in  broad 
and  very  irregularly  outlined  plates,  filling  the  interstices  of 
the  feldspars,  which  are  themselves  in  short  and  stout  forms 
quite  at  variance  with  the  elongated,  lath-shaped  forms  seen  in 
diabases.  This  rule  is,  however,  in  some  cases  reversed,  and 
the  feldspars  occur  in  broad,  irregular  forms  surrounding  the 
more  perfectly  formed  pyroxenes.  Transitions  into  diabase 
structure  are  not  uncommon.  In  rare  instances  the  pyroxenic 
constituents  occur  in  concretionary  aggregates  as  in  the  peculiar 
kugel  gabbro  or  potato  rock  from  Smaalanene,  in  Norway. 
Through  a  molecular  change  of  the  pyroxenic  constituent,  the 
gabbros  pass  into  diorites,  as  do  also  the  diabases. 

Colors.  —  The  prevailing  colors  are  gray  to  nearly  black ; 
sometimes  greenish  through  decomposition. 

Classification.  —  The  rocks  of   this   group  are  divided   into 

(1)  the  true  gabbros  —  that  is,  plagioclase-diallage  rocks  —  and 

(2)  norites,    or    plagioclase-bronzite    and    hypersthene   rocks. 
Both  varieties  are  further  subdivided  according  to  the  presence 
or  absence  of  olivine.     We  then  have  :  — 


THE    DIABASES 


87 


True  gabbro      =  Plagioclase  -f  diallage. 
Olivine  gabbro  =  Plagioclase  -f-  diallage  and  olivine. 
Norite  =  Plagioclase  -f  hypersthene  or  bronzite. 

Olivine  norite  =  Plagioclase  -f  hypersthene  and  olivine. 

Nearly  all  gabbros  contain  more  or  less  rhombic  pyroxene,  and 
hence  pass  by  gradual  transitions  into  the  norites.  Through 
a  diminution  in  the  proportion  of  feldspar  they  pass  into  the 
peroditites,  and  a  like  diminution  in  the  proportion  of  pyroxene 
gives  rise  to  the  so-called  forellenstein.  Hyperite  is  the  name 
given,  by  Tornebohm,  to  a  rock  intermediate  between  normal 
gabbro  and  norite.  Anorthosite,  as  above  indicated,  is  the  name 
given  to  the  granular  varieties  poor  or  quite  lacking  in  pyrox- 
enes. 

(2)    THE   DIABASES 

Diabase,  from  the  Greek  word  &a/3acrt9,  a  passing  over  ;  so 
called  by  Brongniart  because  the  rock  passes  by  insensible 
gradations  into  diorite. 

Chemical  Composition.  —  The  table  below  shows  the  average 
range  in  composition  of  (I  and  II)  the  plutonic  diabase  and 
(III,  IV,  V,  and  VI)  the  effusive  forms  melaphyr  and  basalt. 


CONSTITUENTS 

I 

II 

III 

IV 

V 

VI 

Silica  (Si02)    .... 

53.13% 

45.46  % 

56.52  % 

51.02  % 

57.25% 

46.90  % 

Alumina  (Al2Oa) 

13.74 

19.94 

13.53 

18.86 

16.45 

10.17 

Ferric  iron  (Fe203)  .     . 
Ferrous  iron  (FeO)  . 

1.08 
9.10 

j  15.36 

12.56 

J6.57 

"1  4.  68 

1.67, 
1.77 

1.22 
5.17 

Lime  (CaO)     .... 

9.47 

8.32 

5.31 

7.36 

7.65 

6.20 

Magnesia  (MgQ)  .     .     . 

8.58 

2.95 

2.79 

5.57 

6.74 

20.98 

Potash  (K2O)  .... 

1.03 

3.21 

3.59 

2.10 

1.57 

2.04 

Soda  (Na20)    .... 

2.30 

2.12 

3.71 

2.54 

3.00 

1.16 

Ignition  

0.90 

0.30 

0.81 

2.86 

0.45 

5.42 

Specific  gravity 

2  96 

2  945 

2.86 

I.  Diabase:  Jersey  City,  New  Jersey  (G.  W.  Hawes).  II.  Diabase:  Palmer 
Hill,  Au  Sable  Forks,  New  York  (J.  F.  Kemp).  III.  Melaphyr:  Hockenberg, 
Silesia.  IV.  Melaphyr,  Falgendorf,  Bohemia  (quoted  from  Zirkel's  Lehrbuch 
der  Petrographie).  V.  Quartz  basalt:  Snag  Lake,  California  (J.  S.  Diller). 
VI.  Basalt  (absarokite)  :  near  Bozeman,  Montana  (G.  P.  Merrill). 


Mineral  Composition.  —  The  essential  constituents  of  diabase 
are  plagioclase  feldspar  and  augite,  with  nearly  always  mag- 


88 


HOCKS   FORMED   THROUGH   IGNEOUS   AGENCIES 


iietite  and  apatite  in  microscopic  proportions.  The  common 
accessories  are  hornblende,  black  mica,  olivine,  enstatite,  hyper- 
sthene,  orthoclase,  quartz,  and  titanic  iron.  Calcite,  chlorite, 
hornblende,  and  serpentine  are  common  as  products  of  altera- 
tion. Through  a  molecular  change  known  as  uralitization  the 
augite  not  infrequently  becomes  converted  into  hornblende, 
as  already  described  (p.  40),  and  the  rock  thus  passes  over 
into  diorite.  The  plagioclase  may  be  labradorite,  oligoclase, 
or  anorthite. 

Structure.  —  In   structure  the  diabases    are    holocrystalline. 
Rarely  do  the  constituents  possess  perfect  crystal  outlines,  but 

are  more  or  less  imper- 
fect and  distorted,  owing 
to  mutual  interference  in 
process  of  formation,  the 
granular  hypidiomorphic 
structure  of  Professor 
Rosenbusch.  The  augite 
in  the  typical  forms  oc- 


FIG.  4.  —  Microstructure  of  diabase. 


curs  in  broad  and  sharply 
angular  plates  enclosing 
the  elongated  or  lath- 
shaped  crystal  of  plagio- 
clase, giving  rise  to  a 
structure  known  as  ophi- 
tic.  (See  Fig.  4.)  The 
rocks  are,  as  a  rule,  com- 
pact, fine,  and  homoge- 
neous, though  sometimes  porphyritic  and  rarely  amygdaloidal. 
Colors. —  The  colors  are  sombre,  varying  from  greenish  through 
dark  gray  to  nearly  black,  the  green  color  being  due  to  a  dissemi- 
nated chloritic  or  serpentinous  product  resulting  from  the  alter- 
ation of  the  augite  or  olivine. 

Classification.  —  Two  principal  varieties  are  recognized,  the 
distinction  being  based  upon  the  presence  or  absence  of  the 
mineral  olivine.  We  thus  have:  (1)  diabase  proper  and  (2)  oli- 
vine diabase. 

Many  varietal  names  have  been  given  from  time  to  time  by 
different  authors.  Gumbel  gave  the  name  of  leucophyr  to  a 
very  chloritic,  diabase-like  rock  consisting  of  pale  green  augite 
and  a  saussurite-like  plagioclase.  The  same  authority  gave 


THE   DIABASES  89 

the  name  epidiorite  to  an  altered  diabase  rock  occurring  in 
small  dikes  between  the  Cambrian  and  Silurian  formations 
in  the  Fichtelgebirge,  and  in  which  the  augite  had  become 
changed  to  hornblende.  He  also  designated  by  the  term  pro- 
terobase  a  Silurian  diabase  consisting  of  a  green  or  brown, 
somewhat  fibrous,  hornblende,  reddish  augite,  two  varieties  of 
plagioclase,  chlorite,  ilmenite,  a  little  magnetite,  and  usually  a 
magnesian  mica.  The  name  ophite  has  been  used  by  Pallarson 
to  designate  an  augite  plagioclase  eruptive  rock,  rich  in  horn- 
blende and  epidote,  and  occurring  in  the  Pyrenees.  The 
researches  of  M.  Levy  Kuhn1  and  others  have,  however,  shown 
that  both  hornblende  and  epidote  are  secondary,  resulting  from 
the  augitic  alteration,  and  that  the  rock  must  be  regarded  as 
belonging  to  the  diabase. 

The  Swedish  geologist,  Tornebohm,  gave  the  name  sahlite 
diabase  to  a  class  of  diabasic  rocks  containing  the  pyroxene 
sahlite,  and  which  occurred  in  dikes  cutting  the  granite,  gneiss, 
and  Cambrian  sandstones  in  the  province  of  Smaaland,  and  in 
other  localities.  The  name  teschenite  was  for  many  years  ap- 
plied to  a  class  of  rocks  occurring  in  Moravia,  and  which,  until 
the  recent  researches  of  Rohrbach,  were  supposed  to  contain 
nepheline,  but  which  are  now  regarded  as  merely  varietal  forms 
of  diabase.  Variolite  is  a  compact,  often  spherulitic,  variety 
occurring  in  some  instances  as  marginal  facies  of  ordinary 
diabase.  The  name  eukrite  or  eucrite  was  first  used  by  G.  Rose 
to  designate  a  rock  consisting  of  white  anorthite  and  grayish 
green  augite  occurring  in  the  form  of  a  dike  cutting  the  Car- 
boniferous limestone  of  Carlingford  district,  Ireland.  These 
rocks  were  included  by  Professor  Zirkel  under  the  head  of 
"anorthitgesteine."  The  name  is  now  little  used,  and  rocks 
of  this  type  are  here  included  with  the  diabases. 

The  diabases  are  among  the  most  abundant  and  wide-spread 
of  our  so-called  trap  rocks,  occurring  in  the  form  of  dikes, 
intrusive  sheets,  and  bosses.  They  are  especially  characteristic 
of  the  Triassic  formations  of  the  eastern  United  States.  It 
should  be  noted,  however,  that  many  of  these  Triassic  traps 
have  been  shown  to  be  true  lava  flows,  and  that  on  both  litho- 
logical  and  geological  grounds  such  might  with  propriety  be 
classed  with  the  basalts. 

1  Untersuchungen  liber  pyrenaeische  Ophite,  Inaugural  Dissertation  Univer- 
sitat,  Leipzig,  1881. 


90  KOCKS  FORMED   THROUGH   IGNEOUS   AGENCIES 


(3)   THE   MELAPHYRS  AND   AUGITE  PORPHYRITES 

The  term  melaphyr  is  used  to  designate  a  volcanic  rock 
occurring  in  the  form  of  intrusive  sheets  and  lava  flows,  and 
consisting  essentially  of  a  plagioclase  feldspar,  augite,  and 
olivine,  with  free  iron  oxides  and  an  amorphous  of  porphyry 
base.  The  augite  porphyrites  differ  in  containing  no  olivine. 
The  rocks  of  this  group  are  therefore  the  porphyritic,  effusive, 
forms  of  the  olivine-bearing  and  divine-free  diabases  and 
gabbros. 

Structure.  —  As  above  noted,  they  are  porphyritic  rocks  with, 
in  their  typical  forms,  an  amorphous  base,  are  often  amygda- 
loidal,  and  with  a  marked  flow  structure. 

Colors.  —  In  colors  they  vary  through  gray  or  brown  to  nearly 
black  ;  often  greenish  through  chloritic  and  epidotic  decompo- 
sition. 

Classification  and  Nomenclature.  —  According  as  olivine  is 
present  or  absent,  they  are  divided  primarily  into  melaphyrs 
and  augite  porphyrites,  the  first  bearing  the  same  relation  to 
the  olivine  diabases  as  do  the  quartz  porphyries  to  the  granites, 
or  the  hornblende  porphyrites  to  the  diorites,  and  the  second 
a  similar  relation  to  the  olivine-free  diabases.  The  augite 
porphyrites  are  further  divided  upon  structural  grounds  into 
(1)  diabase  porphyrite,  which  includes  the  varieties  with  holo- 
crystalline  diabase  granular  ground-mass  of  augite,  iron  ores, 
and  feldspars,  in  which  are  embedded  porphyritic  lime-soda 
feldspars,  —  mainly  labradorite,  —  idiomorphic  augites,  and  at 
times  accessory  hornblende  and  black  mica ;  (2)  spilite,  which 
includes  the  non-porphyritic  compact,  sometimes  amygdaloidal 
and  decomposed  forms  such  as  are  known  to  German  petrog- 
raphers  as  dichte  diabase,  diabase  mandelstein  (amygdaloid), 
kalk-diabase,  variolite,  etc.;  (3)  the  true  augite  porphyrite,  in- 
cluding the  normal  porphyritic  forms  with  the  amorphous  base, 
and  (4)  the  glassy  variety  augite  vitrophyrite. 

(4)    THE    BASALTS 

Basalt,  a  very  old  term  used  by  Pliny  and  Strabo  to  designate 
certain  blacks  rocks  from  Egypt,  and  which  were  employed  in 
the  arts  in  early  times.1 

1  Teall,  British  Petrography,  p.  136. 


THE    BASALTS  91 

Mineral  Composition.  —  The  essential  minerals  are  augite  and 
plagioclase  feldspar  with  olivine  in  the  normal  forms;  accessory 
iron  ores  (magnetite  and  ilmenite),  together  with  apatite,  are 
always  present,  and  more  rarely  a  rhombic  pyroxene,  horn- 
blende, black  mica,  quartz,  perowskite,  hauyne  and  nepheline, 
and  minerals  of  the  spinel  group.  Metallic  iron  has  been 
found  as  a  constituent  of  certain  basaltic  rocks  on  Disco  Island, 
Greenland. 

Chemical  Composition.  —  The  composition  is  quite  variable, 
as  shown  by  analyses  in  columns  V  and  VI  on  p.  87.  The  fol- 
lowing shows  the  common  extremes  of  variation :  Silica,  45  % 
to  55  %;  alumina,  10  %  to  18  %;  lime,  7  %  to  14  %;  magnesia, 
3  %  to  10  % ;  oxide  of  iron  and  manganese,  9  %  to  16  % ; 
potash,  0.058  % ;  soda,  2  %  to  5  % ;  loss  by  ignition,  1%  to  5  %  ; 
specific  gravity,  2.85  to  3.10. 

Structure.  —  Basalts  vary  all  the  way  from  clear  glassy  to 
holocrystalline  forms.  The  common  type  is  a  compact  and, 
to  the  unaided  eye,  homogeneous  rock,  with  a  splintery  or 
conchoidal  fracture,  and  showing  only  porphyritic  olivines  in 
such  size  as  to  be  recognizable.  Under  the  microscope  they 
show  a  ground-mass  of  small  feldspar  and  augite  microlites, 
with  perhaps  a  sprinkling  of  porphyritic  forms  of  feldspar, 
augite,  and  olivine,  and  a  varying  amount  of  interstitial  brown- 
ish glass;  the  glass  may  be  wholly  or  in  part  replaced  by  devit- 
rification products,  as  minute  hairs,  needles,  and  granules.  A 
marked  flow  structure  is  often  developed,  the  feldspars  of  the 
ground-mass  having  flowed  around  the  olivine  belonging  to  the 
earlier  period  of  consolidation,  giving  rise  to  an  appearance 
that  may  be  compared  to  logs  in  a  mill  stream,  the  olivines 
representing  small  islands.  Pumiceous  and  amygdaloidal 
forms  are  common. 

Colors.  —  The  prevailing  colors  are  dark,  some  shade  of  gray 
to  perfectly  black.  Red  and  brown  colors  are  also  common. 
Mineralogically  it  will  be  observed  the  basalts  resemble  the 
olivine  diabases  and  melaphyrs,  of  which  they  may  be  regarded 
as  the  younger  equivalents.  Indeed,  in  very  many  cases  it  has 
been  found  impossible  to  ascertain  from  the  study  of  the  speci- 
men alone  to  which  of  the  three  groups  it  should  be  referred,  so 
closely  at  times  do  they  resemble  one  another. 

Classification  and  Nomenclature.  —  In  classifying,  the  varia- 
tions in  crystalline  structure  are  the  controlling  factors.  As, 


92  ROCKS   FORMED   THROUGH   IGNEOUS   AGENCIES 

however,  these  characteristics  are  such  as  may  vary  almost 
indefinitely  in  different  portions  of  the  same  flow,  the  rule  has 
not  been  rigidly  adhered  to  here.  We  thus  have  :  — 

(1)  Dolerite,  including  the  coarse-grained  almost  holocrys- 
talline  variety ;  (2)  anamesite,  including  the  very  compact 
tine-grained  variety,  the  various  constituents  of  which  are  not 
distinguishable  by  the  unaided  eye  ;  (3)  basalt  proper,  which 
includes  the  compact  homogeneous,  often  porphyritic,  variety, 
carrying  a  larger  proportion  of  interstitial  glass  or  devitrifica- 
tion products  than  either  of  the  above  varieties,  and  (4)  tacky  - 
lite,  hyalomelan,  or  hyalobasalt,  which  includes  the  vitreous  or 
glassy  varieties,  the  mass  having  cooled  too  rapidly  to  allow  it  to 
assume  a  crystalline  structure.  These  varieties,  therefore,  bear 
the  same  relation  to  normal  basalt  as  do  the  obsidians  to  the 
liparites.  Other  varieties,  though  less  common,  are  recogniz- 
able and  characterized  by  the  presence  or  absence  of  some 
predominating  accessory  mineral.  We  have  thus  quartz,  horn- 
blende, and  hypersthene  basalt,  etc.  An  olivine-free  variety  is 
also  recognized. 

The  basalts  are  among  the  most  abundant  and  wide-spread  of 
the  younger  eruptive  rocks.  Jn  the  United  States  the}r  are 
found  mainly  in  the  regions  west  of  the  Mississippi  River. 
They  are  eminently  volcanic  rocks,  and  occur  in  the  form  of  lava 
streams  and  sheets,  often  of  great  extent,  and  sometimes  show- 
ing a  characteristic  columnar  structure.  According  to  Rich- 
thofen,  the  basalts  are  the  latest  products  of  volcanic  activity. 
A  quartz-bearing  basalt  has  been  described  by  Mr.  J.  S.  Diller 
as  occurring  at  Snag  Lake,  near  Lassens  Peak,  California,  and 
which  is  regarded  by  him  as  a  product  of  the  latest  volcanic 
eruption  within  the  limits  of  the  state.  This  lava  field  covers 
an  area  of  only  some  three  square  miles,  and  trunks  of  trees 
killed  at  the  time  of  the  eruption  are  still  standing.1 

Under  the  name  of  melilite  basalt  is  included  a  group  of  rocks 
in  which  the  mineral  melilite  is  the  characterizing  constituent, 
with  accessory  augite,  olivine,  nepheline,  biotite,  magnetite, 
perowskite,  and  spinel.  The  normal  structure  is  holocrystal- 
line  porphyritic,  in  which  the  olivine,  augite,  mica,  or  occasion- 
ally the  melilite,  appear  as  porphyritic  constituents.  These  are 
rocks  of  very  limited  distribution,  and  at  present  known  in 
North  America  only  near  Montreal,  Canada.  Professor  Rosen- 
1  Bull.  No.  79,  U.  S.  Geol.  Survey,  1891. 


THE   THERALITE-BASANITE    GROUP  93 

busch,  in  his  latest  work,  separates  this  entirely  from  the  basalts, 
and  considers  it  in  a  group  by  itself  under  the  name  of  Melilite 
Rocks. 

6.     THE  THERALITE-BASANITE  GROUP 

This  is  a  small,  and  so  far  as  now  known,  comparatively  in- 
significant group  of  rocks,  representatives  of  which  are  confined 
to  limited  and  widely  separated  areas.  They  are  described  as 
below  :  — 

(1)    THE   THERALITES 

This  name,  derived  from  the  Greek  word  Orjpav,  to  seek 
eagerly,  is  given  by  Professor  Rosenbusch  to  a  class  of  intru- 
sive rocks  consisting  essentially  of  plagioclase  feldspar  and 
nepheline,  and  which  are  apparently  the  plutonic  equivalents  of 
the  tephrites  and  basanites. 

The  group  is  founded  by  Professor  Rosenbusch  upon  certain 
rocks  occurring  in  dikes  and  laccolites  in  the  Cretaceous  sand- 
stones of  the  Crazy  Mountains  of  Montana,  and  described  by 
Professor  J.  E.  Wolff,1  of  Harvard  University. 

Mineral  Composition.  —  The  essential  constituents  as  above 
noted  are  nepheline  and  plagioclase  with  accessory  augite, 
olivine,  sodalite,  biotite,  magnetite,  apatite  and  secondary  horn- 
blende, and  zeolitic  minerals. 

Chemical  Composition.  —  The  chemical  composition  of  a  sam- 
ple from  near  Martinsdale,  as  given  by  Professor  Wolff,  is  as 
follows:  Silica,  43.175%;  alumina,  15.236%;  ferrous  oxide, 
7.607  %  ;  ferric  oxide,  2.668  %  ;  lime,  10.633  %  ;  magnesia, 
5.810%;  potash,  4.070%;  soda,  5.68%;  water,  3.571%; 
sulphuric  anhydride,  0.94  %. 

Structure.  —  The  rocks  are  holocrystalline  granular  through- 
out. 

Colors.  —  These  are  dark  gray  to  nearly  black. 

The  theralites,  so  far  as  known,  have  an  extremely  limited 
distribution,  and  in  the  United  States  have  thus  far  been  re- 
ported only  from  Gordon's  Butte  and  Upper  Shields  River  basin 
in  the  Crazy  Mountains  of  Montana. 

1  Notes  on  the  Petrography  of  the  Crazy  Mountains  and  other  localities  in 
Montana,  by  J.  E.  Wolff.  Neues  Jahrb.  fur.  Min.,  etc.,  1885,  I,  p.  69. 


94 


ROCKS  FORMED  THROUGH  IGNEOUS  AGENCIES 


(2)  THE  TEPHRITES  AND  BASANITES 

Mineral  Composition.  —  The  essential  constituent  of  the  rocks 
of  this  group  as  given  by  Rosenbusch  are  a  lime-soda  feldspar 
and  nepheline  or  leucite,  either  alone  or  accompanied  by  augite. 
Olivine  is,  essential  in  basanite.  Apatite,  the  iron  ores,  and 
rarely  zircon  occur  in  both  varieties.  Common  accessories  are 
sanidin,  hornblende,  biotite,  hauyne,  melanite,  perowskite,  and 
a  mineral  of  the  spinel  group. 

Chemical   Composition.  —  The  following  is  the  composition  of 

(I)  a  nepheline  tephrite  from  Antao,  Pico  da  Cruz,  Azores,  and 

(II)  a    nepheline   basanite   from   San    Antonio,    Cape    Verde 
Islands,  as  given   by  Roth.1 


CONSTITUENTS 

I 

II 

Silica  (SK>2)          .    .              .             . 

47  44  % 

43  09  °L 

2371 

17.45 

6.83 

18.99 

Iron  protoxide  (FeO) 

353 

Magnesia  (MgO)                 .              .     . 

1  95 

463 

Lime  (CaO)      

647 

976 

Soda  (Na20)     

6.40 

502 

Potash  (K2O) 

334 

1  81 

Water  (H20) 

1  73 

033 

Structure.  —  The  rocks  of  this  group  are  as  a  rule  porphyritic 
with  a  holocrystalline  ground-mass,  though  sometimes  there  is 
present  a  small  amount  of  amorphous  interstitial  matter  or 
base;  at  times  amygdaloidal. 

Colors.  —  The  colors  are  dark,  some  shade  of  gray  or  brownish. 

Classification  and  Nomenclature.  —  According  to  their  vary- 
ing mineral  composition  Rosenbusch  divides  them  into  :  — 

Leucite  tephrite       =  Leucite,  augite,  plagioclase  rocks. 

Leucite  basanite      =  Leucite,  augite,  plagioclase  and  olivine  rocks. 

Nepheline  tephrite  =  Nepheline,  plagioclase  rocks. 

Nepheline  basanite  =  Nepheline,  plagioclase  and  olivine  rocks. 

The  group,  it  will  be  observed,  stands  intermediate  between 
the  true  basalts  and  the  nephelinites  to  be  noted  later.  Their 
distribution,  so  far  as  now  known,  is  quite  limited. 

1  Abhandlungen  der  Konig.  Akad.  der  Wissenschaften  zu  Berlin,  1884,  p.  64. 


THE   PERIDOTITE-LIMBURGITE   GROUP 


95 


7.     THE  PERIDOTITE-LIMBURGITE  GROUP 

This  and  the  following  groups  include  eruptive  rocks  in 
which  neither  quartz  nor  feldspars  of  any  kind  longer  appear 
as  essential  constituents,  and  which  are  therefore  very  low  in 
silica,  causing  them  to  be  classed  as  ultrabasic.  Although  in 
most  cases  comparatively  insignificant  as  rock  masses,  they  are 
peculiarly  interesting  as  mineral  aggregates,  and  even  more  on 
account  of  the  character  of  their  alteration  products.  The 
peridotites  are  further  of  interest  in  presenting  the  nearest 
homologues  to  meteorites  of  any  of  our  terrestrial  rocks.  The 
group  includes  the  plutonic  peridotites  '(serpentine  in  part),  and 
effusive  picrite  porphyrites  and  limburgites.  In  detail  these  are 
as  below  :  — 

(1)    THE   PERIDOTITES 

Peridotite,  so  called  because  the  mineral  peridot  (olivine)  is 
the  chief  constituent. 

Mineral  Composition.  —  The  essential  constituent  is  olivine 
associated  nearly  always  with  chromite  or  picotite  and  the  iron 
ores.  The  common  accessories  are  one  or  more  of  the  ferro- 
magnesian  silicate  minerals  augite,  hornblende,  enstatite,  and 
black  mica ;  feldspar  is  also  present  in  certain  varieties  and 
more  rarely  apatite,  garnet,  sillimanite,  perowskite,  and  pyrite. 


CONSTITUENTS 

I 

II 

III 

IV 

V 

VI 

Silica  (Si02)      .... 

41.58o/0 

43.84  % 

39.103% 

42.94% 

38.01  % 

45.68  % 

Alumina  (AloOs.)  .     .     . 

0.14 

1.14 

4.94 

10.87 

5.32 

6.28 

Magnesia  (MgO)    .     .     . 

49.28 

44.33 

29.176 

16.32 

23.29 

34.76 

Lime  (CaO)      .... 

0.11 

1.71 

3.951 

9.07 

4.11 

2.15 

Iron  sesquioxide(Fe203) 

8.76 

4.315 

3.47 

6.70 

9.12 

Iron  protoxide  (FeO)     . 

7.49 

.... 

11.441 

10.14 

4.92 



Chrome  oxide  (Cr203)  . 



0.42 

0.436 

.... 



0.26 

Manganese  (MnO)    .     . 

.... 

0.12 

0.276 

Trace 

.... 

Potash  (K2O)    .... 

.... 

.... 

Trace 

0.15 

0.22 

Soda  (Na20) 

0.90 

4.15 

Nickel  oxide  (NiO) 

0.34 

Water  and  ignition    .     . 

1.72 

1.06 

5.669 

6.09 

10.60 

1.21 

Specific  gravity      .     .     . 



3.287 

2.93 

2.88 

2.83 

3.269 

I.  Dunite  :  Macon  County,  North  Carolina.  II.  Saxonite :  St.  Paul's  Rocks, 
Atlantic  Ocean.  III.  Picrite :  Nassau,  Germany.  IV.  Hornblende  picrite  : 
Ty  Cross,  Anglesia.  V.  Picrite  :  Little  Deer  Isle,  Maine.  VI.  Lherzolite  : 
Monte  Rossi,  Piedmont. 


96 


ROCKS   FORMED   THROUGH   IGNEOUS  AGENCIES 


Chemical  Composition.  —  The  chemical  composition  varies 
somewhat  with  the  character  and  abundance  of  the  prevailing 
accessory.  The  preceding  table  shows  the  composition  of 
several  typical  varieties. 

Structure.  —  The  structure  as  displayed  in  the  different 
varieties  is  somewhat  variable.  In  the  dunite  it  is  as  a  rule 
even  crystalline  granular,  none  of  the  olivines  showing  perfect 
crystal  outlines.  In  the  picrites  the  augite  or  hornblende  often 

occurs  in  the  form  of 
broad  plates  occupying 
the  interstices  of  the  oli- 
vines and  wholly  or  par- 
tially enclosing  them,  as 
in  the  hornblende  pic- 
rite  of  Stony  Point,  New 
York.  The  saxonites  and 
Iherzolites  often  show 
a  marked  porphyritic 
structure  produced  by 
the  development  of  large 
pyroxene  crystals  in  the 
fine  and  evenly  granular 
ground-mass  of  olivines. 
(See  Fig.  5,  as  drawn  by 
Dr.  G.  H.  Williams.) 
The  rocks  belong  to  the 
class  designated  as  hypidiomorphic  granular  by  Professor 
Rosenbusch;  that  is,  rocks  composed  only  in  part  of  minerals 
showing  crystal  faces  peculiar  to  their  species. 

Colors.  —  The  prevailing  colors  are  green,  greenish  gray,  yel- 
lowish green,  dark  green  to  black. 

Nomenclature  and  Classification.  —  Mineralogically  and  geo- 
logically it  will  be  observed  the  peridotites  bear  a  close  resem- 
blance to  the  olivine  diabases  and  gabbros,  from  which  they 
differ  only  in  the  absence  of  feldspars.  Indeed,  Professor  Judd 
has  shown  that  the  gabbros  and  diabase  both,  in  places,  pass  by 
insensible  gradations  into  peridotites  through  a  gradual  dimi- 
nution in  the  amount  of  their  feldspathic  constituents.  Dr. 
Wadsworth  would  extend  the  term  peridotite  to  include  rocks 
of  the  same  composition,  but  of  meteoric  as  well  as  terrestrial 
origin,  the  condition  of  the  included  iron,  whether  metallic  or 


FIG.  5.  —  Microstructure  of  porphyritic  Iherzo- 
lite,  partly  altered  into  serpentine. 


THE   PERIDOTITES  97 

as  an  oxide,  being  considered  by  him  as  non-essential,  since 
native  iron  is  also  found  occasionally  in  terrestrial  rocks,  as 
the  Greenland  basalts  and  some  diabases. 

In  classifying  the  peridotites  the  varietal  distinctions  are 
based  upon  the  prevailing  accessory  mineral.  We  thus  have :  — 

Dunite,  consisting  essentially  of  olivine  only. 

Saxonite,  consisting  essentially  of  olivine  and  enstatite. 

Picrite,  consisting  essentially  of  olivine  and  augite. 

Hornblende  picrite,  consisting  essentially  of  olivine  and  hornblende. 

Wehrlite  (or  eulysite),  consisting  essentially  of  olivine  and  diallage. 

Lherzolite,  consisting  essentially  of  olivine,  enstatite,  and  augite. 

The  name  Dunite  was  first  used  by  Hochstetter  and  applied 
to  the  olivine  rock  of  Mount  Dun,  New  Zealand.  Saxonite 
was  given  by  Wadsworth,  rocks  of  this  type  being  prevalent  in 
Saxony.  The  same  rock  has  since  been  named  Harzburgite  by 
Rosenbusch.  The  name  Lherzolite  is  from  Lake  Lherz  in  the 
Pyrenees. 

The  peridotites  are,  as  a  rule,  highly  altered  rocks,  the  older 
forms  showing  a  more  or  less  complete  transformation  of  their 
original  constituents  into  a  variety  of  secondary  minerals,  the 
olivine  going  over  into  serpentine  or  talc  and  the  augite  or 
hornblende  into  chlorite.  The  most  common  result  of  this 
alteration  is  the  rock  serpentine,  the  transformation  taking 
place  through  the  hydration  of  the  olivine  and  the  liberation  of 
free  iron  oxides  and  chalcedony.  (See  Fig.  5.)  Recent  inves- 
tigations have  shown  that  a  large  share  of  the  serpentinous 
rocks  were  thus  originated.  The  chemistry  of  the  process  has 
been  already  discussed  under  the  head  of  olivine,  p.  24. ' 

Since  in  this  process  of  hydration  the  combined  iron  becomes 
converted  into  the  sesquioxide  form,  and  the  calcium  of  the 
lime-magnesian  silicates  separates  out  in  large  part  as  free  cal- 
cite,  or  as  mixed  carbonates  of  lime  and  magnesia,  so  these  ser- 
pentinous rocks  are  rarely  uniform  in  color  or  composition. 
The  prevailing  color  is  some  shade  of  green,  though  not  infre- 
quently brown,  yellow,  red,  or  nearly  black.  Through  the 
presence  of  still  unaltered  grains  of  pyroxene,  many  varieties 
are  porphyritic.  The  rock  is  almost  universally  badly  jointed, 
an  evident  necessary  accompaniment  to  the  alteration,  and  into 
these  joints  have  filtered  the  lime  or  magnesia  carbonate  solu- 
tions, where,  depositing  their  load,  they  have  formed  the  numer- 


98  ROCKS   FORMED   THROUGH   IGNEOUS  AGENCIES 

ous  white,  yellow,  and  greenish  veins  with  which  the  stone  is 
traversed.  Many  varieties  indeed,  like  the  rosso  de  Levante, 
verde  di  Pegli,  and  verde  di  Crenora  of  Italy,  are  but  breccias  of 
serpeiitinous  fragments  cemented  by  calcareous  and  ferruginous 
cements.1 

It  is,  perhaps,  as  yet  too  early  to  state  definitely  that  all  peri- 
dotites  are  eruptive.  In  many  instances  their  eruptive  nature 
is  beyond  dispute.  Others  are  found  in  connection  with  the 
crystalline  schists,  so  situated  as  to  suggest  that  they  may  them- 
selves be  metamorphic. 

(2)    THE   PICRITE  PORPHYRITES 

Under  this  head  is  placed  a  small  group  of  rocks  so  far  as 
now  known  very  limited  in  their  distribution,  and  which  are 
regarded  as  the  effusive  forms  of  the  plutonic  picrites,  as  bear- 
ing the  same  relation  to  these  rocks  as  do  the  melaphyrs  to  the 
olivine  diabases.  The  essential  constituents  are  therefore  oli- 
vine  and  augite  with  accessory  apatite,  iron  ores,  and  other 
minerals  mentioned  as  occurring  in  the  true  picrites.  Struct- 
urally they  differ  from  these  rocks  in  presenting  an  amorphous 
base  rather  than  being  crystalline  throughout.  Rocks  of  this 
type  are  supposed  to  have  had  an  important  bearing  on  the 
origin  of  the  diamond,  the  diamond-bearing  rocks  of  South 
Africa  being  picrite  porphyrite  (kimberlite)  cutting  highly 
carbonaceous  shales.  An  examination  of  the  Kentucky  peri- 
dotite  locality,  where  the  same  rock  occurs  under  quite  similar 
conditions,  failed  to  show  that  similar  results  have  been  there 
produced,  a  fact  which  is  supposed  to  be  due  in  part  to  the 
small  amount  of  carbonaceous  matter  in  the  surrounding  shales. 

The  group  is  very  limited,  and  is  represented  in  the  United 
States  only  in  Elliott  County,  Kentucky ;  Pike  County,  Arkan- 
sas ;  Syracuse,  Onondaga  County,  New  York. 

(3)    THE   LIMBURGITES 

This  is  a  small  group  of  lavas  described  by  Rosenbusch  in 
1872  as  occurring  at  Limburg,  or  the  Kaiserstuhl  in  the  Rhine. 
The  essential  constituents  are  augite  and  olivine  with  the  usual 
iron  ores.  Structurally  the  rock  is  so  far  as  known  never  holo- 
crystalline,  but  glassy  and  porphyritic.  The  composition  of  the 

1  See  the  Stones  for  Building  and  Decoration,  Wiley  &  Sons,  New  York. 


THE   PYROXENITE-AUGITITE    GROUP  99 

Prussian  limburgite  is  given  as  below.      So  far  as  known,  the 
group  has  no  representatives  in  the  United  States. 


CONSTITUENTS 

PER  CENT 

Silica  (Si02)     

42  24 

18  66 

Iron  sesquioxide  (Fe203)     

7  45 

Magnesia  (M°'0)  

12  27 

Lime  (CaO)      

11  76 

Soda  (Na«0)     

4  02 

Potash  (K2O)    

1  08 

Water  (H20)    

3  71 

99.19 

8.    THE  PYROXENITE-AUGITITE  GROUP 

Here  are  included  a  small  group  of  eruptive  rocks  differing 
from  the  last  mainly  in  the  absence  of  olivine  as  an  essential 
constituent.  They  are  represented,  so  far  as  now  known,  only 
by  the  plutonic  pyroxenites  and  effusive  augitites. 

(1)    THE   PYROXENITES 

Pyroxenite,  a  term  applied  by  Dr.  Hunt  to  certain  rocks  con- 
sisting essentially  of  minerals  of  the  pyroxene  group,  and  which 
occurred  both  as  intrusive  and  as  beds  or  nests  intercalated  with 
stratified  rocks.  The  author  here  follows  the  nomenclature  and 
classification  adopted  by  Dr.  G.  H.  Williams.1 

Mineral  Composition.  —  The  essential  constituents  are  one  or 
more  minerals  of  the  pyroxene  group,  either  orthorhombic  or 
monoclinic.  Accessory  minerals  are  not  abundant  and  limited 
mainly  to  the  iron  ores  and  minerals  of  the  hornblende  or  mica 
groups. 

Chemical  Composition. — The  following  analyses  serve  to  show 
the  variations  which  are  due  mainly  to  the  varying  character 
of  the  pyroxenic  constituents :  — 

1  American  Geologist,  Vol.  VI,  July,  1890,  pp.  35-49. 


100 


ROCKS   FORMED   THROUGH   IGNEOUS   AGENCIES 


CONSTITUENTS 

I 

II 

in 

Silica  (SiO2)  

50.80  % 

53.98  % 

55  14  % 

Alumina  (AlgOs) 

340 

132 

066 

Chrome  oxide  (Cr208)     
Ferric  oxide  (Fe203)  

0.32 
1.39 

0.53 
1.41 

0.25 
348 

Ferrous  oxide  (FeO) 

8  11 

390 

4  73 

Manganese  (MnO) 

017 

021 

003 

Lime  (CaO)    

12.31 

1547 

839 

22.77 

22  59 

2666 

Soda  (Na20) 

Trace 

030 

Potash  (K20)      

Trace 

Water  (H20)  

052 

083 

038 

Chlorine  (Cl)      

024 

023 

.... 

100.03% 

100.24  % 

100.25% 

I.  Hypersthene-diallage  rock :  Johnny  Cake  Road,  Baltimore  County,  Mary- 
land. II.  Hypersthene-diallage  rock :  Hebbville  post-office,  Baltimore  County, 
Maryland.  III.  Bronzite-diopside  rock  from  near  Webster,  North  Carolina. 

Structure.  —  The  pyroxenites  are  holocrystalline  granular 
rocks,  at  times  evenly  granular  and  saccharoidal,  or  again 

porphyritic,    as     in    the 
websterite 
Carolina. 


North 


from 

The  micro- 
scopic structure  of  this 
rock  is  shown  in  Fig.  6 
from  the  original  draw- 
ing by  Dr.  Williams. 

Colors.  —  The  colors 
are,  as  a  rule,  greenish  or 
bronze. 

Classification  and  No- 
menclature. — The  pyrox- 
enites, it  will  be  observed, 
differ  from  the  peridotites 
only  in  the  lack  of  olivine. 
Following  Dr.  Williams's 

.bio.  6.  —  Microstructure  of  websterite,  Webster.  , 

North  Carolina.  nomenclature,   we     have 

the    varieties    diallagite, 

bronzitite,  and  hypersthenite,  according  as  the  mineral  diallage, 
brohzite,  or  hypersthene  forms  the  essential  constituent.  Web- 
sterite is  the  name  given  to  the  enstatite-diopside  variety,  such 


AUGITITE 


101 


as  occurs  near  Webster,  North  Carolina,  and  hornblendite  to 
the  hornblende-augite  variety.  The  pyroxenites  rank,  in  geo- 
logical importance,  next  to  the  peridotites.  Through  processes 
of  hydration  and  other  chemical  changes,  these  rocks  pass  into 
amphibolic  and  steatitic  masses  to  which  the  name  soapstone 
or  potstone  is  not  infrequently  applied.  These  are  dark  gray 
or  greenish  rocks,  soft  enough  to  be  readily  cut  with  a  knife 
and  with  a  pronounced  soapy  or  greasy  feeling  ;  hence  the 
name  soapstone.  The  name  potstone  was  given  on  account  of 
their  having  been  utilized  for  making  rude  pots,  for  which  their 
softness  and  fireproof  properties  render  them  well  qualified. 
Although  it  is  commonly  stated  in  the  text-books  that  soap- 
stone  is  a  compact  form  of  steatite  or  talc,  few  are  even  ap- 
proximately pure  forms  of  this  mineral,  but  all  contain  varying 
proportions  of  chlorite,  mica,  and  tremolite,  together  with  per- 
haps unaltered  residuals  of  pyroxene,  granules  of  iron  ore,  iron 
pyrites,  quartz,  and,  in  seams  and  veins,  calcite  and  magnesian 
carbonates.  The  variation  in  chemical  composition  is  shown  in 
the  following  analyses,  I  being  that  of  a  compact,  homogeneous- 
appearing,  quite  massive  variety  from  Alberene,  in  Albemarle 
County,  Virginia,  and  II  one  from  Francestown,  New  Hamp- 
shire. 


CONSTITUENTS 

I 

II 

Silica  (Si02)      

39.06% 

42.43% 

Alumina  (AloOs)  

12.84 

6.08 

Ferric  and  ferrous  iron  (Fe203)  and  (FeO)      .... 
Lime  (CaO) 

12.90 
5.98 

13.07 
3.27 

Magnesia  (IVIgO) 

22.76 

25.71 

Potash  (KoO)                                                         .         .     . 

0.19 

0.32 

Soda  (Na20)                             .     .                   

0.11 

0.16 

Ignition    

6.56 

8.45 

100.40% 

99.49% 

(2)    AUGITITE 

The  effusive  form,  augitite,  differs  from  the  pyroxenite  proper 
mainly  on  structural  grounds.  In  common  with  many  lavas  it 
has  a  glassy  base,  in  which  are  embedded  the  crystals  of  augite 
and  iron  ores.  The  composition  of  an  augitite  from  the  Cape 
Verde  Islands,  as  given  by  Roth,  is  as  below :  — 


102  ROCKS  FORMED    THROUGH   IGNEOUS   AGENCIES 


CONSTITUENTS 

PER  CENT 

Silica  (SK>2)                                                            .         ... 

41  83 

Alumina  (A^Oa)        .              .                       .         

18  60 

16.11 

Magnesia  (MsjO)                                                                 . 

4  98 

Lime  (CaO)                                                           .         ... 

11  83 

Soda  (Na20)          

4  70 

Potash  (K2O)    

2.47 

Water  (H20) 

0  91 

101.43 

9.    THE  LEUCITE-NEPHELINE  ROCKS 

Under  this  head  are  grouped  two  small  but  interesting  groups 
of  effusive  rocks,  having,  so  far  as  known,  no  exact  equivalent 
among  the  plutonics,  and  characterized  by  the  presence  of  leu- 
cite  or  nepheline  as  essential  constituents  and  which  here  seem 
to  play  the  role  of  feldspars.  In  detail  they  are  as  below:  — 


(1)    THE   LEUCITE   ROCKS 

Mineral  Composition.  —  The  essential  constituent  is  leucite 
and  a  basic  augite.  A  variety  of  accessories  occur,  including 
biotite,  hornblende,  iron  ores,  apatite,  olivine,  plagioclase,  nephe- 
line, melilite,  and  more  rarely  garnets,  hauyne,  sphene,  chromite, 
and  perowskite.  Feldspar  as  an  essential  fails  entirely. 

Chemical  Composition.  — The  average  chemical  composition  as 
given  by  Blaas1  is  as  follows  :  Silica,  48.9  %  ;  alumina,  19.5  %  ; 
iron  oxides,  9.2%;  lime,  8.9%;  magnesia,  1.9%;  potash, 
6.5%;  soda,  4.4%. 

Structure.  —  The  rocks  of  this  group  are,  as  a  rule,  fine 
grained  and  only  slightly  vesicular,  presenting  to  the  unaided 
eye  little  to  distinguish  them  from  the  finer-grained  varieties 
of  ordinary  basalt. 

Colors.  —  The  prevailing  colors  are  some  shades  of  gray, 
though  sometimes  yellowish  or  brownish. 

Classification  and  Nomenclature.  —  The  varietal  distinctions 
are  based  upon  the  presence  or  absence  of  the  mineral  olivine 

1  Katechismus  der  Petrographie,  p.  117. 


THE   NEFHELINE   ROCKS 


103 


and  upon  structural  grounds  and  various  minor  characteristics. 
We  have  the  olivine-free  variety  leucitite  and  the  olivine-holding 
variety  leucite  basalt. 

These  rocks  have  also  a  very  limited  distribution,  and,  so  far 
as  known,  are  found  within  the  limits  of  the  United  States  only 
at  the  Leucite  Hills,  Wyoming. 


(2)   THE   NEPHELINE   ROCKS 

Mineral  Composition.  —  These  rocks  consist  essentially  of 
nepheline  with  a  basaltic  augite  and  accessory  sanidin,  pla- 
gioclase,  mica,  olivine,  leucite,  minerals  of  the  sodalite  group, 
magnetite,  apatite,  perowskite,  and  melanite. 

Chemical  Composition.  —  Below  is  given  the  composition  of 
(I)  a  nephelinite  from  the  Cape  Verde  Islands,  and  (II)  a 
nepheline  basalt  from  the  Vogelsberg,  Prussia.1 


CONSTITUENTS 

I 

II 

Silica  (Si02)      

46  95  % 

42  37  °/ 

Alumina  (Al20s)   

21.59 

8  88 

Iron  sesquioxide  (I^Og)     
Iron  protoxide  (FeO) 

8.09 

11.26 

7  80 

Magnesia  (MsjO)              .     .                         . 

2  49 

13  01 

Lime  (CaO)                          ... 

7  97 

10  93 

Soda  (Na20)                    .... 

8.93 

4  51 

Potash  (K20)     ...               .     . 

2.04 

1.21 

Water  (H20)     

2.09 

0.34 

3.103 

Colors.  —  The  prevailing  colors  are  various  shades  of  gray 
to  nearly  black. 

Structure.  —  Structurally  they  are  porphyritic,  with  a  holo- 
crystalline  or  in  part  amorphous  base,  usually  fine  grained  and 
compact,  at  times  amygdaloidal. 

Classification  and  Nomenclature.  — These  rocks  differ  from  the 
basalts,  which  they  otherwise  greatly  resemble,  in  that  they  bear 
the  mineral  nepheline  in  place  of  feldspar.  Based  upon  the 
presence  or  absence  of  olivine,  we  have,  first,  nepheline  basalt, 


1  Roth,  Abhandl.  der  Konig.  Preus.  Akad.  der  Wiss.  zu  Berlin,  1884. 


104  ROCKS   FORMED   THROUGH   IGNEOUS   AGENCIES 

and  second,  nephelinite.  The  name  nepheline  dolerite  has  been 
given  in  some  cases  to  the  coarser,  holocrystalline,  olivine- 
bearing  varieties. 

Like  the  leucite  rocks,  the  members  of  this  group  are  some- 
what limited  in  their  distribution. 


II.     AQUEOUS    ROCKS 
1.     ROCKS  FORMED  THROUGH  CHEMICAL  AGENCIES 

This  comparatively  small,  though  by  no  means  unimportant, 
group  of  rocks  comprises  those  substances  which,  having  once 
been  in  a  condition  of  aqueous  solution,  have  been  deposited  as 
rock  masses  either  by  cooling,  evaporation,  by  a  diminution  of 
pressure,  or  by  direct  chemical  precipitation.  It  also  includes 
the  simpler  forms  of  those  produced  by  chemical  changes  in 
pre-existing  rocks.  Water,  when  pure  or  charged  with  more 
or  less  acid  or  alkaline  material,  and  particularly  when  acting 
under  great  pressure,  is  an  almost  universal  solvent.  Thus, 
heated  alkaline  waters,  permeating  the  rocks  of  the  earth's 
crust  at  great  depths  below  the  surface,  are  enabled  to  dis- 
solve from  them  various  mineral  matters  with  which  they  come 
in  contact.  On  coming  to  the  surface  or  flowing  into  crevices, 
the  pressure  is  diminished,  or  evaporation  takes  place,  and  the 
water,  no  longer  able  to  carry  its  load,  deposits  it  wholly  or  in 
part  as  vein  material  or  a  surface  coating.  In  other  cases  alka- 
line or  acid  water,  bearing  mineral  matters,  may,  in  course  of 
their  percolations,  be  brought  in  contact  with  neutralizing  solu- 
tions, and  these  dissolved  materials  be  thus  deposited  by  direct 
precipitation.  In  these  various  ways  were  formed  the  rocks 
here  described.  It  will  be  observed  that  the  various  members 
of  the  group  are  composed  mainly  of  minerals  of  a  single  species 
only. 

This  group  cannot,  however,  be  separated  by  any  sharp  lines 
from  that  which  is  to  follow,  inasmuch  as  many  rocks  are  not 
the  product  of  a  single  agency,  acting  alone,  but  are  rather  the 
result  of  two  or  more  combined  processes.  This  is  especially  the 
case  with  the  limestones.  It  is  safe  to  assume  that  few  of  these 
are  due  wholly  to  accumulations  of  calcareous,  organic  remains, 
but  are,  in  part  at  least,  chemical  precipitates,  as  is  well  illus- 
trated by  the  oolitic  varieties. 

105 


106  AQUEOUS   ROCKS 

According  to  their  chemical  nature,  the  group  is  divided 
into  (1)  Oxides,  (2)  Carbonates,  (3)  Silicates,  (4)  Sulphates, 
(5)  Phosphates,  (6)  Chlorides,  and  (7)  the  Hydrocarbon  Com- 
pounds. 

(l)  OXIDES 

Here  are  included  those  rocks  consisting  essentially  of  oxygen 
combined  with  a  base,  though  usually  other  constituents  are 
present  as  impurities. 

Hematite.  —  Anhydrous  sesquioxide  of  iron.  Fe2O3  =  oxy- 
gen, 30  % ;  iron,  70  %.  In  nature  nearly  always  more  or  less  im- 
pure through  the  mechanical  admixture  of  argillaceous  silicates 
or  calcareous  matter,  manganese  oxides,  sulphur,  phosphates, 
etc.  Several  forms  are  recognized,  the  distinction  being  based 
mainly  upon  physical  properties.  Specular  hematite  is  a  mica- 
ceous or  foliated  variety  with  a  black,  metallic,  often  splendent 
lustre ;  this  variety  is  mainly  a  metamorphic  form,  and  prop- 
erly should  be  classed  with  the  metamorphic  rocks.  Compact, 
columnar,  fibrous,  and  earthy  forms  also  occur,  the  latter  often 
known  as  ochre,  as  are  similar  forms  of  limonite.  Although 
classified  here  under  the  head  of  aqueous  rocks,  it  does  not 
follow  that  the  hematites  have  all  originated  in  precisely  the 
same  manner.  To  a  limited  extent  the  specular  variety  is  found 
about  volcanic  craters  and  fumaroles,  where  it  was  originally 
deposited  by  a  process  of  sublimation.  Through  a  process  of 
oxidation,  beds  of  magnetic  iron  become  locally  altered  into 
hematite,  giving  rise  to  pseudomorphous  granular,  octahedral, 
and  dodecahedral  forms,  to  which  the  name  martite  is  given. 
Many  extensive  beds  undoubtedly  arise  from  the  dehydration 
by  dynamic  agencies  —  the  folding  and  metamorphosing  of  the 
enclosing  rocks  —  of  beds  of  limonite.  Others,  like  the  fossil 
and  oolitic  ores  of  the  Clinton  formations,  arise  in  part  from  a 
process  of  chemical  precipitation  and  subsequent  segregation, 
the  ore  being  originally  disseminated  throughout  a  ferruginous 
limestone,  and  having  accumulated  as  an  insoluble  residue  as 
the  lime  carbonate  was  carried  away  through  the  action  of  car- 
bonated waters.  The  extensive  hematite  deposits  of  the  Lake 
Superior  region  of  Michigan  are  regarded  as  oxidation  prod- 
ucts from  pre-existing  carbonates  (siderite),  the  oxide  having 
been  precipitated  from  solution  in  synclinal  troughs,  and  subse- 
quently crystallized  by  metamorphism.1  The  ores  of  the  Mesabi 

1  Van  Hise  Monograph  XIX,  U.  S.  Geol.  Survey,  1892. 


FIG.  1.   Botryoidal  hematite. 


FIG.  2.   Clay-iron  stone  septarian  nodule. 


OXIDES  107 

range,  on  the  other  hand,  are  regarded  by  at  least  one  writer 
as  having  originated  through  a  somewhat  complicated  process 
of  oxidation  and  metasomatosis,  whereby  a  pre-existing  glauco- 
nitic  rock  (a  ferruginous  silicate)  became  converted  into  an 
admixture  of  free  iron  oxide  and  silica,  the  one  or  the  other, 
according  to  the  intermittent  character  of  the  permeating  solu- 
tions, being  leached  out  and  redeposited  at  no  great  distance  in 
a  fair  condition  of  purity.1  A  discussion  of  this  subject  belongs 
more  properly  to  economic  geology,  and  need  not  be  dwelt 
upon  further  here. 

Limonite  (Brown  Hematite).  —  Iron  sesquioxide  plus  water. 
H6Fe2O6  +  Fe2O3.  An  earthy  or  compact  dark  brown,  black, 
or  ochreous-yellow  rock,  containing,  when  pure,  about  two- 
thirds  its  weight  of  pure  iron.  It  occurs  in  beds,  veins,  and 
concretionary  forms,  associated  with  rocks  of  all  ages,  and 
forms  a  valuable  ore  of  iron.  (See  Fig.  1,  PL  9.)  On  the  bot- 
toms of  lakes,  bogs,  and  marshes  it  often  forms  in  extensive 
deposits,  where  it  is  known  as  bog-iron  ore.  The  formation  of 
these  deposits  is  described  as  follows :  Iron  is  widely  diffused 
in  rocks  of  all  ages,  chiefly  in  the  form  of  (1)  the  protoxide, 
which  is  readily  soluble  in  waters  impregnated  with  carbonic 
or  other  feeble  acids,  or  (2)  the  peroxide,  which  is  insoluble  in 
the  same  liquids.  Water  percolating  through  the  soils  becomes 
impregnated  with  these  acids  from  the  decomposing  organic 
matter,  and  then  dissolves  the  iron  protoxide  with  which  it 
comes  in  contact.  On  coming  to  the  surface  and  being  exposed 
to  the  air,  as  in  a  stagnant  lake  or  marsh,  this  dissolved  oxide 
absorbs  more  oxygen,  becoming  converted  into  the  insoluble 
sesquioxide,  which  floats  temporarily  on  the  surface  as  an  oil- 
like,  iridescent  scum.  Finally  this  sinks  to  the  bottom,  where 
it  gradually  becomes  aggregated  as  a  massive  iron  ore.  This 
same  ore  may  also  form  through  the  oxidation  of  pyrite,  or 
beds  of  ferrous  carbonate.  At  the  Ktaadn  Iron  Works,  in 
Piscataquis  County,  Maine,  the  ferrous  salt  as  it  oxidizes  is 
brought  to  the  surface  by  water  and  deposited  as  a  coating 
over  the  leaves  and  twigs  scattered  about,  forming  thus  beauti- 
fully perfect  casts,  or  fossils. 

Pyrolusite,  Psilomelane,  and  Wad. —  These  are  names  given 
to  the  anhydrous  and  more  or  less  hydrated  forms  of  manganese 

1  J.  E.  Spurr,  Bull.  No.  10,  Geol.  and  Nat.  Hist.  Survey  of  Minnesota,  1894. 


108  AQUEOUS   ROCKS 

oxides,  and  which,  though  wide  in  their  distribution,  are  found 
in  such  abundance  as  to  constitute  rock  masses  in  comparative 
rarity.  The  origin  of  such  deposits  is  at  times  somewhat  ob- 
scure. In  all  cases  they  are  doubtless  secondary.  The  original 
source  of  the  material  appears  to  have  been  the  manganiferous 
silicates  of  Archsean  and  more  recent  eruptive  rocks,  whence  it 
was  derived  by  leaching,  being  transported  in  the  form  of 
soluble  salts  and  finally  precipitated  as  oxide  or  carbonate,  the 
latter  being  subsequently  converted  into  oxide.  The  deposits 
which  are  of  sufficient  extent  to  be  of  commercial  value  occur 
as  a  rule  in  residual  clays,  as  interbedded  strata  in  shales  and 
sandstones,  or  as  occupying  superficial  seams  and  joints,  and 
in  the  form  of  pockets  and  nests.  True  fissure  veins  of  man- 
ganese oxide  are  not  known.  It  is  often  associated  with  the 
form  of  limonite  known  as  bog-iron  ore,  and,  apparently,  has 
been  deposited  contemporaneously. 

Beauxite  (so  called  from  Beaux,  near  Aries,  France)  is  the 
name  given  to  a  somewhat  indefinite  mixture  of  alumina  and 
iron  oxides,  and  occurring  in  the  form  of  compact  concretion- 
ary grains  of  a  dull  red,  brown,  or  nearly  white  color,  and 
also  in  compact  and  earthy  forms.  The  mode  of  occurrence  of 
the  mineral  is  somewhat  variable.  At  Beaux  and  several  other 
localities  it  occurs  in  pockets  in  limestone,  and  also  in  beds 
alternating  with  limestones,  sandstones,  and  clays  belonging 
to  the  Cretaceous  period.  In  the  Puy-de-D6me  the  beds  rest 
directly  upon  gneiss,  and  are  overlaid  by  basalt.  At  Oberhes- 
sen,  Germany,  the  mineral  occurs  in  rounded  masses  embedded 
in  clay,  as  is  also  the  case  at  Vogelsberg.  In  America,  beaux- 
ite  has  been  found  in  Alabama,  Georgia,  and  Arkansas.  In 
Alabama  and  Georgia  it  occurs  in  beds  of  irregular  extent, 
associated  with  limestones  of  Upper  Cambrian  age  (the  Knox 
dolomite);  in  Arkansas  the  deposits  are  Tertiary. 

The  origin  of  the  beauxite  is  somewhat  obscure.  It  has  been 
argued  that  the  beds  at  Beaux,  and  those  of  Var,  are  deposits 
from  mineral  springs.  Those  of  the  Puy-de-D6me,  the  West- 
erwald,  Vogelsberg,  and  of  Ireland,  on  the  other  hand,  are 
regarded  as  derived  from  basalt  by  a  metasomatic  process. 
The  Alabama  and  Georgia  deposits,  like  those  of  Beaux,  are 
regarded  as  of  chemical  origin.1 

1  See  resume  of  the  subject,  by  R.  L.  Packard,  in  Mineral  Resources  of  the 
United  States  for  1891. 


OXIDES 


109 


According  to  C.  Willard  Hayes,1  the  prevailing  rocks  of  this 
region  are  dolomites  underlaid  by  aluminous  shales.  It  is 
assumed  that  heated  waters,  in  their  passage  upward  from 
greater  depths,  have  oxidized  the  iron  sulphides  of  the  shale, 
giving  rise  to  sulphates  of  iron,  of  alumina,  and  the  double 
sulphates  of  alumina  and  potash.  As  the  ascending  water, 
carrying  these  salts  in  solution,  passes  through  the  dolomite,  it 
becomes  charged  with  calcium  carbonate,  which  causes  the  pre- 
cipitation of  the  aluminum  salts  in  the  concretionary,  pisolitic 
form  so  characteristic. 

Beauxite  has,  of  late,  come  to  be  of  considerable  economic 
value  as  an  ore  of  aluminum,  and  as  a  source  of  alum,  in  place 
of  clay. 

The  material  from  various  sources  varies  greatly  in  chemical 
composition,  as  shown  by  the  following  analyses :  — 


CONSTITUENTS 

I 

II 

III 

IV 

V 

Silica  (Si02) 

2.8% 

1  10  % 

21  08  % 

2  80  % 

10  38  °/ 

Alumina  (A1203)     .     .     . 
Iron  sesquioxide  (Fe2C>3)  . 
Water  (H20)  

57.6 
25.3 

10.08 

50.92 
15.70 
27.75 

48.92 
2.14 
23.41 

52.21 
13.50 

27.72 

55.64 
1.95 
27.62 

Titanium  oxide  (TiO2) 

3.1 

3.20 

2.52 

3.52 

3.50 

I.  Beaux,  France.     II.  Vogelsberg,  Germany.     III.  Jacksonville,  Alabama. 
IV.  Floyd  County,  Georgia.     V.  Pulaski  County,  Arkansas. 

•  Silica.  —  Silica,  as  has  been  already  noted  under  the  head  of 
rock-forming  minerals,  is  one  of  the  most  abundant  constituents 
of  the  earth's  crust.  In  its  various  forms,  which  are  sufficiently 
extensive  to  constitute  rock  masses,  it  is  always  of  chemical 
origin,  that  is,  results  by  deposition  from  solution,  by  precipi- 
tation, or  evaporation,  as  noted  above.  Varietal  names  are 
given  to  the  deposits,  dependent  upon  their  structure,  method 
of  formation,  color,  and  degree  of  purity.  Siliceous  sinter, 
geyserite,  or  fiorite  is  the  name  given  to  the  nearly  white, 
often  soft  and  friable,  hydrated  varieties  formed  on  the  evapo- 
ration of  the  siliceous  waters  of  hot  springs  and  geysers,  or 
through  the  eliminating  action  of  algous  vegetation,  as  de- 
scribed by  W.  H.  Weed  in  the  reports  of  the  United  States 

1  Trans.  Am.  Inst.  of  Mining  Engineers,  February,  1894. 


110  AQUEOUS  ROCKS 

Geological  Survey.1  The  material  is,  in  reality,  an  impure 
form  of  opal.  Throughout  the  geyser  regions  of  the  Yellow- 
stone Park,  Iceland,  and  New  Zealand,  the  sinter  has  been 
deposited  as  a  comparatively  thin  crust  over  the  surface,  or  in 
the  form  of  cones  about  the  throats  of  the  geysers.  The  vari- 
eties of  silica  known  as  opal  are  hydrous  forms  occurring  in 
veins  and  pockets,  in  a  variety  of  rocks.  Not  infrequently  it 
forms  the  replacing  material  in  silicified  or  "  petrified  "  woods. 
In  the  old  lake  beds  of  the  Madison  valley,  Montana,  may  not 
infrequently  be  found  large  logs  composed  wholly  of  this  mate- 
rial, no  sign  of  organic  matter  remaining,  but  yet  with  the 
woody  structure  beautifully  preserved. 

The  origin  of  these  silicified  logs,  so  far  as  it  has  been  traced, 
appears  to  have  been  somewhat  as  follows :  The  water  which 
permeated  the  lake  beds  in  which  these  logs  lay,  was  more  or 
less  alkaline,  and  carried  small  amounts  of  silica  in  solution. 
As  the  logs  slowly  decayed,  there  were  given  off  minute  quan- 
tities of  organic  acids  which,  neutralizing  the  alkaline  water, 
caused  a  gradual  precipitation  of  the  silica,  building  up  thus  an 
exact  cast  of  the  decaying  structure.  Chalcedony  is  the  trans- 
lucent, massive,  c^ptocrystalline  variety  of  silica  occurring 
mainly  in  cavities  in  older  rocks,  where  it  has  been  deposited 
by  infiltration.  It  is  a  common  secondary  product  formed 
during  the  decomposition  of  many  rocks,  and,  like  opal,  not 
infrequently  forms  the  petrifying  medium  of  fossil  woods  and 
other  organisms.  Not  infrequently,  also,  it  occurs  in  continu- 
ous layers  of  several  inches  in  thickness,  interstratified  with 
limestone,  as  may  be  seen  in  the  walls  of  the  Wyandotte  caves 
in  southern  Indiana,  or,  more  rarely,  in  beds  from  2  to  8  feet 
thick,  interstratified  with  coal  and  fire-clay,  as  at  the  well- 
known  "  Flint  Ridge  "  of  Licking  County,  Ohio.  Such  depos- 
its are  considered  to  be  due  to  accumulations  of  the  siliceous 
tests  of  diatoms.  Flint  is  a  variety  of  chalcedony  formed  by 
segregation  in  chalky  limestone,  and  is  composed,  in  part,  of 
the  broken  and  partially  dissolved  spicules  of  sponges,  and 
the  siliceous  casts  of  infusoria.  The  source  of  the  silica  is, 
doubtless,  the  sponge  spicules  above  noted  and  diatomaceous 
remains.  Chert  is  an  impure  flint  containing  not  infrequently 
fossil  nummulitic  remains,  and  with  sometimes  a  pronounced 

1  9th  Ann.  Rep.  U.  S.  Geol.  Survey,  1887-88.  See  also  Bischof  s  Chemical 
and  Physical  Geology,  Vol.  I,  pp.  184-200. 


CARBONATES  111 

oolitic  structure.  It  occurs  in  rounded,  nodular,  concretionary 
masses  interbedded  with  limestones,  particularly  Palaeozoic  vari- 
eties, and  doubtless  originated  as  did  the  flints  in  the  chalky 
limestones.  Jasper  is  a  dull  or  bright  red,  or  yellow  variety 
of  chalcedony  containing  alumina,  and  owing  its  color  to  iron 
oxides.  It  is  sometimes  used  in  jewellery. 

The  name  novaculite  is  frequently  given  to  very  fine-grained 
and  compact  quartz  rocks,  such  as  are  suitable  for  hones.  As 
commonly  used,  the  name  is  made  to  include  rocks  of  widely 
different  origin,  some  of  which  are  evidently  chemical  precipi- 
tates, while  others  are  indurated  clastic  or  schistose  rocks.  The 
well-known  novaculites  of  Arkansas  are  clear  white  masses  of 
chalcedonic  silica,  containing  scattering  quartz  granules,  minute 
grains  of  garnet,  and  numerous  small  rhomboidal  cavities  which 
seemingly  were  once  occupied  by  crystals  of  calcite  or  dolomite. 
Opinions  differ  as  to  the  origin  of  this  rock.  Owen.1  regarded 
it  as  a  sandstone  metamorphosed  by  percolating  hot  water. 
Branner2  looked  upon  it  as  a  metamorphosed  chert ;  Griswold,3 
as  a  chemical  deposit  in  the  form  of  a  siliceous  slime  on  a  sea- 
bottom,  while  Rutley4  argues  that  it  is  but  a  siliceous  replace- 
ment of  beds  of  dolomite  or  dolomitic  limestone.  It  seems 
probable  that  the  views  of  Branner  or  Rutley  are  the  most 
nearly  correct. 

Quartz  is  a  massive  form  of  crystalline  silica  occurring  in 
veins,  disseminated  granules,  and  pockets  in  rocks  of  all  kinds 
and  all  ages.  It  is  one  of  the  most  wide-spread  and  commonest 
of  minerals,  and  is  frequently  quarried  and  crushed  for  abrasive 
purposes  or  use  in  pottery  manufacture.  It  is  not  infrequently 
of  a  pink  or  rose  color  from  metallic  oxides.  It  is  a  common 
gangue  of  ores  of  the  precious  metals,  particularly  of  gold. 
Lydian  stone  is  an  exceedingly  hard  impure  quartz  rock,  of  a 
black  color  and  splintery  fracture.  It  was  formerly  much 
used  in  testing  the  purity  of  precious  metals. 

(2)  CARBONATES 

Water  carrying  small  amounts  of  carbonic  acid  readily  dis- 
solves the  calcium  carbonate  of  rocks  with  which  it  comes  in 

1  2d  Rep.  Geological  Reconnaissance  of  Arkansas,  1860. 

2  Ann.  Rep.  Geol.  Survey  of  Arkansas,  Vol.  I,  1886,  p.  49. 

3  Ann.  Rep.  Geol.  Survey  of  Arkansas,  Vol.  Ill,  1890. 

4  Quarterly  Journal  Geological  Society  of  London,  August,  1894. 


112 


AQUEOUS   ROCKS 


contact ;  on  evaporation  and  through  loss  of  a  portion  of  the 
carbonic  acid,  this  is  again  deposited.  In  this  way  are  formed 
numerous  and  at  times  extensive  deposits,  to  which  are  given 
varietal  names  dependent  upon  their  structure  and  the  special 
conditions  under  which  they  originated.  Gale  sinter  or  tufa  is 
a  loose  friable  deposit  made  by  springs  and  streams  either  by 
evaporation  or  through  intervention  of  algous  vegetation.  Such 
are  often  beautifully  arborescent  and  of  a  snow-white  color,  as 
seen  at  the  Mammoth  Hot  Springs  of  the  Yellowstone  National 
Park.  Somewhat  similar  deposits  are  formed  by  springs  in 
Virginia,  California,  Mexico,  New  Zealand.  Others,  like  those 
from  Niagara  Falls,  New  York,  and  Soda  Springs,  Idaho,  were 
formed  by  the  deposition  of  the  lime  on  leaves  and  twigs,  form- 
ing beautifully  perfect  casts  of  these  objects. 

Tufa  deposits  of  peculiar  imitative  shapes  have  been  described 
by  Mr.  I.  C.  Russell  of  the  United  States  Geological  Survey, 
as  formed  by  the  evaporation  of  the  waters  of  Pyramid  Lake, 

Nevada.  Oolitic  and  pi- 
solitic  limestones  are  so 
called  on  account  of  their 
rounded,  fish  -  egg  -  like 
structure,  the  word  oolite 
being  from  the  Greek 
word  (oov,  an  egg.  (See 
PI.  12.)  These  are  in 
part  chemical  and  in  part 
mechanical  deposits.  The 
water  in  the  lakes  and 
seas  in  which  they  were 
formed  became  so  satu- 
rated that  the  lime  was 
deposited  in  concentric 
coatings  about  the  grains 
of  calcareous  sand  on  the 
bottom,  and  finally  the  little  granules  thus  formed  became 
cemented  into  firm  rock  by  the  further  deposition  of  lime  in 
the  interstices.  This  structure  will  be  best  understood  by 
reference  to  Fig.  7.  Rocks  of  this  nature  are  now  forming 
along  the  beaches  of  Pyramid  Lake.  Concerning  the  occur- 
rence of  these  Mr.  Russell  writes  :  — 

"  Among  The  Needles  the  rocky  capes  are  connected  by  cres- 


FIG.  7.  —  Microstructure  of  oolitic  limestone. 


CARBONATES  113 

cent-shaped  beaches  of  clean,  creamy  sands,  over  which  the 
summer  surf  breaks  with  soft  murmurs.  These  sands  are  oolitic 
in  structure,  and  are  formed  of  concentric  layers  of  carbonate 
of  lime  which  is  being  deposited  near  where  the  warm  springs 
rise  in  the  shallow  margin  of  the  lake.  In  places  these  grains 
have  increased  by  continual  accretion  until  they  are  a  quarter 
of  an  inch  or  more  in  diameter,  and  form  gravel,  or  pisolite,  as 
it  would  be  termed  by  mineralogists.  In  a  few  localities  this 
material  has  been  cemented  into  a  solid  rock,  and  forms  an 
oolitic  limestone  sufficiently  compact  to  receive  a  polish.  No 
more  attractive  place  can  be  found  for  the  bather  than  these 
secluded  coves,  with  their  beaches  of  pearl-like  pebbles,  or  the 
rocky  capes,  washed  by  pellucid  waters,  that  offer  tempting 
leaps  to  the  bold  diver." 

Such  forms  as  these  may  or  may  not  show  a  nucleus.  It 
seems  safe  to  assume  that  such  a  nucleus,  at  first,  in  all  cases 
existed,  though  it  may  be  in  microscopic  dimensions  only. 

Travertine  is  a  compact  and  usually  crystalline  deposit  formed, 
like  the  tufas,  by  waters  of  springs  and  streams.  The  traver- 
tines are  often  beautifully  veined  and  colored  by  metallic  oxides 
and  form  some  of  the  finest  marbles.  Such  are  the  so-called 
"onyx  marbles"  of  Mexico  and  Arizona.1 

Stalactite  and  stalagmite  are  the  names  given  to  the  deposits 
formed  from  the  roofs  and  on  the  floors  of  caves  ;  water,  perco- 
lating through  the  limestone  roof,  by  virtue  of  the  carbonic  acid 
it  contains,  dissolves  out  a  small  amount  of  the  lime,  which,  on 
evaporation,  is  again  deposited  either  as  pendent  cones  from 
the  ceiling,  or  as  massive  and  pillar-like  forms  upon  the  floor. 
The  pendants  are  known  as  stalactites ;  the  corresponding 
growths  upon  the  floor  as  stalagmites.  Stalactite  and  stalag- 
mite sometimes  meet,  forming  thus  continuous  pillars,  or  col- 
umns extending  from  floor  to  ceiling.  The  lime  of  these 
deposits,  it  may  be  said,  is  as  a  rule  in  the  form  of  calcite, 
though  sometimes,  as  in  the  old  portions  of  the  Wyandotte 
caves  in  Indiana,  it  is  aragonite.  The  so-called  "  oriental  ala- 
baster "  of  the  ancients  is  a  stalagmitic  deposit  derived  in  part 
from  crevices  and  pockets  in  the  Eocene  limestones  of  the  Nile 
valley. 

Magnesite,  a  carbonate  of  magnesia,  occurs  frequently  as  a 

1  The  Onyx  Marbles,  Ann.  Rep.  U.  S.  National  Museum  for  1893.  Also 
Stones  for  Building  and  Decoration,  Wiley  £  Sons,  New  York,  2d  ed.,  p.  120. 


114  AQUEOUS   ROCKS 

secondary  mineral  in  the  form  of  veins  in  serpentinous  rocks, 
but  rarely  itself  forms  rock  masses  of  any  importance.  Rhodo- 
chrosite,  a  carbonate  of  manganese,  sometimes  occurs  in  rock 
masses,  but  is  found  most  commonly  in  the  form  of  veins  asso- 
ciated with  ores  of  silver,  lead,  or  copper. 

Another  carbonate,  less  common  than  that  of  lime,  but  which 
sometimes  occurs  in  such  quantities  as  to  constitute  true  rock 
masses,  is  siderite,  or  carbonate  of  iron.  A  common  form  of 
this  is  dull  brownish  or  nearly  black  in  color,  very  compact  and 
impure,  containing  varying  amounts  of  calcareous,  clayey,  and 
organic  matter.  In  this  condition  it  is  found  in  stratified  beds 
and  in  the  shape  of  rounded  and  oval  nodules,  or  concretions, 
which  are  called  clay -ironstone  nodules,  septaria,  and  sphcero- 
siderite.  (See  Fig.  2,  PI.  9.)  These  septarian  nodules  are 
often  beautifully  veined  with  calcite,  and  when  cut  and  polished 
form  not  undesirable  objects  of  ornamentation.  Other  forms  of 
siderite  are  massive,  coarsely  crystalline,  and  of  a  nearly  white 
or  yellowish  color,  becoming  brownish  on  exposure.  Pure  sider- 
ite yields  about  48  %  metallic  iron,  and  is  of  value  as  an  ore. 

(3)    SILICATES 

Silica,  combined  with  magnesia  and  water,  gives  rise  to  an 
interesting  group  of  serpentinous  and  talcose  substances,  which 
are  often  sufficiently  abundant  to  constitute  rock  masses.  Pure 
serpentine  consists  of  about  equal  parts  of  silica  and  magnesia, 
with  from  12  to  13  %  of  water.  It  is  a  compact,  amorphous,  or 
colloidal  rock,  soft  enough  to  be  cut  with  a  knife,  with  a  slight 
greasy  feeling  and  lustre,  and  of  a  color  varying  from  dull 
greenish  and  almost  black,  through  all  shades  of  yellow,  brown- 
ish, and  red.  It  also  occurs  in  fibrous  and  silky  forms,  filling 
narrow  veins  in  the  massive  rocks,  and  is  known  as  amianthus, 
or  chrysotile.  These  fibres,  when  sufficiently  long,  are  used  for 
the  manufacture  of  fireproof  material,  and  the  mineral  is  com- 
mercially confounded  with  asbestos,  a  fibrous  variety  of  amphi- 
bole.  It  is  very  doubtful  if  serpentine  is  ever  an  original 
rock  ;  it  is  rather  an  alteration  product  of  other  and  less  stable 
magnesian  minerals.  Here  will  be  considered  only  those  which 
have  originated  by  a  series  of  chemical  changes  known  as  meta- 
somatosis,  a  process  of  indefinite  substitution  and  replacement, 
in  simple  mineral  aggregates  occurring  associated  with  the 


SILICATES 


115 


older  metamorphic  rocks.  Such  are  the  serpentines  derived 
from  non-aluminous  pyroxenes,  like  those  of  Montville,  New 
Jersey,  and  Moriah,  New  York,  and  those  from  Easton,  Penn- 
sylvania, derived  from  a  massive  tremolite  rock.  The  analyses 
given  below  will  serve  to  illustrate  the  chemical  changes  which 
occur  in  this  process  of  metasomatosis,  I  being  that  of  a  nearly 
white  pyroxene,  and  II  that  of  the  serpentine  derived  therefrom. 


CONSTITUENTS 

I 

II 

Silica  (8162)                                         

54.215  °L 

42  38  % 

Magnesia  (MgO)             .          

19.82 

42.14 

Lime  (CaO) 

24  71 

0  00 

Alumina  (AloOa^ 

0  59 

0  07 

Ferric  oxide  (Fe20s^ 

0.20 

0  97 

Ferrous  oride  (FeO) 

0.27 

0  17 

Ignition  (H20)      .                                   

0.14 

14.20 

99.945% 

99.85  % 

The  pyroxene,  it  should  be  observed,  occurs  in  nodular 
masses  in  a  crystalline  granular  dolomite.  Various  stages  of 
the  process  are  shown  in  Fig.  8,  in 
which  the  white  and  gray  central  por- 
tions are  nucleal  masses  of  unchanged 
pyroxenes,  surrounded  by  the  darker 
crusts  of  secondary  serpentine.1  Ser- 
pentine as  an  alteration  product  of  the 
mineral  chondrodite  is  also  known  to 
occur,  though  this  form  is  less  common. 
At  Brewster,  New  York,  are  extensive 
deposits  of  this  nature.  (See  further 
on,  p.  158.) 

Several   varieties   of    serpentine   are 

popularly  recognized.    Precious  or  noble     Fm  g  _  Pyroxene  partially 
serpentine  is  simply  a  very  pure  com-          altered  to  serpentine, 
pact  variety    of    a    deep    oil-yellow    or 

green  color.  Amianthus,  or  chrysotile,  as  noted  above,  is  the 
name  given  to  the  fibrous  variety.  Williamsite  is  a  deep  bright 
green,  translucent,  and  somewhat  scaly  variety,  occurring  asso- 

1  See  On  the   Serpentine   of   Montville,  New  Jersey,  Proc.   U.    S.  National 
Museum,  Vol.  XI,  1888,  p.  105. 


116  AQUEOUS   ROCKS 

elated  with  the  chrome  iron  deposits  in  Fulton  township,  Lan- 
caster County,  Pennsylvania.  Deiveylite  is  a  hard,  translucent 
variety  occurring  in  veins  in  altered  dunite  beds.  Bowenite  is 
a  pale  green  variety  forming  veins  in  limestone  at  Smithfield, 
Rhode  Island.  Picrolite,  marmolite,  and  retinolite  are  varieties  of 
minor  importance.  Serpentine  alone,  or  associated  with  calcite 
and  dolomite,  forms  a  beautiful  marble,  to  which  the  names  verd 
antique,  ophite,  and  ophiolite  are  given.  The  so-called  Eozoon 
Canadense,  a  supposed  fossil  rhizopod,  is  a  mixture  of  serpen- 
tine and  calcite  or  dolomite.  The  name  serpentine  is  from  the 
Latin  serpentinus,  a  serpent,  in  allusion  to  its  green  color  and 
often  mottled  appearance. 

Those  serpentines  which  were  derived  from  basic  eruptives, 
or  complex  metamorphic  rocks  are  described  with  those  rocks 
with  which,  in  their  unaltered  state,  they  would  naturally  be 
grouped. 

The  mineral  steatite,  or  talc,  when  pure,  differs  from  ser- 
pentine in  containing  63.5  %  of  silica,  31.7%  of  magnesia,  and 
4.8  %  of  water.  Its  common  form  is  that  of  white  or  greenish 
inelastic  scales,  forming  an  essential  constituent  of  the  talcose 
schists.  As  is  the  case  with  serpentine,  it  sometimes  results 
from  the  alteration  of  eruptive  magnesian  rocks,  such  as  the 
pyroxenites,  and  rarely  occurs  as  a  direct  result  of  precipita- 
tion. It  will  be  described  more  fully  under  the  head  of  schists 
and  pyroxenites.  Rensselaerite  is  a  closely  related  rock  of  a 
white  or  gray  color,  found  in  St.  Lawrence  County,  New  York. 
Its  composition  is  essentially  that  of  talc. 

Pyrophyllite,  or  agalmatolite,  is  a  hydrous  silicate  of  alumina, 
somewhat  harder  than  talc,  which  it  otherwise  resembles,  and 
which  is  used  in  making  slate  pencils  and  small  images.  It 
occurs  in  a  schistose  form  in  the  Deep  River  region  of  North 
Carolina. 

Kaolin,  also  a  hydrous  silicate  of  alumina,  is  a  chemical 
product  in  that  it  is  a  residue  left  by  the  chemical  decomposi- 
tion of  the  feldspars.  These  minerals,  as  explained  elsewhere, 
consist  of  silicates  of  alumina  and  lime,  with  more  or  less  of 
the  alkalies  potash  and  soda,  and  iron  oxides.  In  the  process 
of  decomposition  new  compounds  are  formed,  the  more  soluble 
of  which  are  leached  out,  leaving  the  less  soluble  silicates, 
including  kaolin,  behind  in  a  condition  of  more  or  less  purity. 
The  mineral  is  of  great  value  for  fictile  purposes,  and  is  de- 


SULPHATES  117 

scribed  more  fully  under  the  head  of  argillaceous  fragmental 
rocks. 

(4)    SULPHATES 

Gypsum.  —  The  rock  gypsum  is  chemically  a  hydrous  sul- 
phate of  lime,  that  is  to  say,  consists  of  sulphur,  lime,  and 
water,  and  in  the  proportion  of  32.6  parts  of  lime  and  20.9 
parts  of  water,  combined  with  46.5  parts  of  sulphur  trioxide. 
When  crystallized,  the  mineral  is  nearly  colorless  and  trans- 
parent, and  splits  readily  into  thin,  inelastic  sheets.  The  com- 
pact massive  varieties  are  white,  gray  to  black,  and  sometimes 
pink  from  various  impurities.  The  most  characteristic  feature 
is  its  softness,  which  is  such  that  it  can  be  readily  cut  with  a 
knife  or  even  by  the  thumbnail. 

Four  varieties  of  gypsum  are  recognized :  (1)  The  common 
massive  form,  dull  in  color  and  often  more  or  less  impure; 
(2)  the  pure  white,  fine-grained  variety,  alabaster;  (3)  the 
fibrous  variety,  satin  spar ;  and  (4)  the  broadly  foliated,  trans- 
parent variety,  selenite,  so  called  from  the  Greek  word  o-eXe^e, 
the  moon,  in  allusion  to  its  soft  and  pleasing  lustre. 

The  following  is  an  analysis  of  a  commercial  gypsum  from 
Ottawa  County,  Ohio,  as  given  by  Professor  Orton  :  l — 

Lime  (CaO) 32.52% 

Sulphuric  acid  (S03) 45.56 

Water  (H20) 20.14 

Magnesia  (MgO) 0.56 

Alumina  (A1203) 0.16 

k  Insoluble  residue  0.68 


99.62  % 

Gypsum  occurs  mainly  associated  with  stratified  rocks,  and 
is  regarded  as  a  chemical  deposit  resulting  from  the  evapora- 
tion of  waters  of  inland  seas  and  lakes ;  it  may  also  originate 
through  the  decomposition  of  sulphides  and  the  action  of  the 
resultant  sulphuric  acid  upon  limestone  ;  through  the  mutual 
decomposition  of  the  carbonate  of  lime  (limestone)  and  the  sul- 
phates of  iron,  copper,  and  other  metals  ;  through  the  hydration 
of  anhydrite  ;  and  through  the  action  of  sulphurous  vapors  and 
solutions  from  volcanoes  upon  the  rocks  with  which  they  come 
in  contact.  According  to  Dana,2  the  gypsum  deposits  in  western 

1  Geology  of  Ohio,  1888,  Vol.  VI,  p.  700. 

2  Manual  of  Geology,  p.  234. 


118  AQUEOUS   ROCKS 

New  York  do  not  form  continuous  layers  in  the  strata,  but  lie 
in  embedded,  sometimes  nodular,  masses  in  limestones.  In  all 
such  cases  this  authority  says  the  gypsum  is  the  product  of  the 
action  of  sulphuric  acid  from  springs  upon  the  limestone.  "  The 
sulphuric  acid,  acting  on  the  carbonate  of  lime,  drives  off  its 
carbonic  acid  and  makes  sulphate  of  lime  or  gypsum ;  and  this 
is  the  true  theory  of  its  formation  in  New  York."  W.  C.  Clarke, 
however,  regards  it  as  a  product  of  deposition  from  solution  in 
sea- water.1 

The  gypsum  deposits  of  northern  Ohio  form  apparently  con- 
tinuous beds  over  thousands  of  square  miles,  and  are  regarded 
by  Professors  Newberry  and  Orton  as  deposits  from  the  evapo- 
ration of  landlocked  seas  at  the  same  time  as  was  the  rock-salt 
which  overlies  it. 

Geological  Age  and  Mode  of  Occurrence.  —  As  may  be  readily 
inferred  from  what  has  gone  before,  beds  of  gypsum  have 
formed  at  many  periods  of  the  earth's  history,  and  are  still 
forming  wherever  proper  conditions  exist. 

In  New  York  there  are  extensive  deposits  belonging  to  the 
Salina  period  of  the  Upper  Silurian.  In  Ohio,  gypsum  asso- 
ciated with  limestones  and  shales  of  Lower  Helderbergage  occur 
over  areas  comprising  thousands  of  square  miles.  The  follow- 
ing section  of  beds  in  Ottawa  County,  this  state,  will  serve  to 
show  the  conditions  under  which  the  rock  may  occur  :  — 

Drift  clays 12  to  14  feet 

Gray  rock  carrying  impure  gypsum - .  5  to  14  feet 

Blue  shale %  to  14  feet 

Boulder  bed  carrying  gypsum  embedded  in  shaly  limestone  .     .     .  o  to  14  feet 

Blue  limestone 1  to  14  feet 

Main  gypsum  bed 7  to  14  feet 

Gray  limestone 1  to  14  feet 

Gypsum     . 3  to    5  feet 

Anhydrite  is  an  anhydrous  variety  of  calcium  sulphate  some- 
what less  common  than  gypsum.  Barite,  or  heavy  spar,  the 
sulphate  of  barium,  also  occurs  in  nature,  but  less  abundantly 
than  the  calcium  sulphates.  It  is  found  commonly  in  con- 
nection with  metallic  ores  (silver,  lead,  and  zinc),  or  as  a 
secondary  mineral  associated  with  limestone,  sometimes  in 
distinct  veins,  or,  as  in  southwest  Virginia,  filling  irregular 
fractures  in  certain  beds  of  the  Cambrian  limestones,  or  in 

i  Bull.  New  York  State  Museum,  Vol.  Ill,  No.  1,  1893. 


PHOSPHATES  119 

part  replacing  the  limestone  itself.  It  is  easily  distinguished 
from  coarsely  crystalline  calcite,  for  which  it  might  possibly 
be  mistaken,  by  its  weight,  the  specific  gravity  being  about 
4.5  as  against  2.7  for  the  latter. 

(5)  PHOSPHATES 

The  mineral  apatite,  a  phosphate  of  lime,  as  already  noted,  is 
a  common  accessory,  in  the  form  of  small  crystals,  in  crystal- 
line rocks  of  all  ages,  both  metamorphic  and  eruptive.  In 
rare  instances,  as  among  certain  Laurentian  rocks  of  Canada,  it 
occurs  in  coarsely  granular  aggregates  of  a  green  or  pinkish 
color  and  of  such  dimensions  as  to  constitute  true  rock  masses. 
Here  we  have  to  do,  however,  more  with  the  amorphous,  fibrous, 
or  concretionary  forms  to  which  the  name  phosphorite  is  com- 
monly applied.  These  occur  nearly  if  not  quite  altogether  as 
secondary  products,  due  to  the  leaching  out  of  phosphatic  mate- 
rial from  older  rocks,  and  its  redeposition  in  clefts  and  cavities 
at  lower  levels.  It  is  thus  that  the  phosphorites  of  Estre- 
madura,  Spain,  are  accounted  for.  From  these  very  pure, 
semi-crystalline  masses,  to  the  amorphous  nodular  and  earthy 
forms,  such  as  are  found  in  the  eastern  Carolinas  and  in  Flor- 
ida, there  are  no  well-defined  lines  of  demarcation.  All  have 
resulted  apparently  either  from  the  leaching  out  of  the  phos- 
phate as  above,  or  from  the  dissolving  and  carrying  away  of  the 
lime  carbonate  in  a  phosphatic  limestone,  leaving  the  phosphatic 
material  to  accumulate  as  a  residual  product.  Some  of  the  latter 
products,  like  the  phosphatic  sandstones  of  the  Carolinas,  might 
with  equal  propriety  be  classed  with  the  fragmental  rocks,  as 
are  the  residual  clays.  (See  p.  151.) 

(6)    CHLORIDES 

Sodium  chloride,  or  common  salt,  is  one  of  the  most  wide- 
spread constituents  of  the  earth's  crust,  and  from  the  standpoint 
of  human  comfort  a  most  important  constituent  as  well.  The 
theoretically  pure  mineral  consists  of  66.6  parts  of  sodium  and 
39.4  parts  of  chlorine,  though  in  nature  it  is  almost  univer- 
sally contaminated  with  chlorides,  sulphates,  and  carbonates 
of  potassium,  calcium,  and  magnesium,  together  with  oxides  of 
iron  and  aluminum.  A  large  number  of  analyses  of  rock-salts 


120  AQUEOUS   BOOKS 

from  world-wide  sources  show  them  to  range  from  94  to  99  % 
sodium  chloride.  The  pure  mineral  is  white  in  color,  but 
shows  often  yellow,  red,  or  purplish  hues  due  to  iron  oxides  or 
organic  matter.  When  crystallizing  freely  from  solution,  it 
ordinarily  assumes  the  form  of  a  cube,  the  faces  being  frequently 
cavernous  or  hopper-shaped  ;  rarely  it  occurs  in  octahedrons, 
and  occasionally  in  fibrous  forms.  Sodium  chloride  in  solution 
is  an  almost  universal  constituent  of  carbonated  waters,  though 
often  in  but  the  merest  traces.  Its  prevailing  solid  form  is  that 
of  coarsely  granular  aggregates  constituting  the  so-called  rock- 
salt,  the  beds  of  which  are  often  of  such  thickness  and  extent 
as  to  constitute  true  rock  masses  and  entitle  them  to  considera- 
tion here.  These  rock  masses  are  invariably  products  of  depo- 
sition from  solution,  a  deposition  brought  about  through  the 
evaporation  of  saline  waters  in  enclosed  lakes  or  seas.  They 
are  not  limited  to  any  particular  geological  period,  but  are  to  be 
found  wherever  suitable  conditions  have  existed  for  their  for- 
mation and  preservation.  Some  of  the  more  important  beds 
now  known  belong  to  either  the  Upper  Silurian,  Carboniferous, 
Triassic,  or  Tertiary  ages,  and  vary  in  thickness  from  a  mere 
film  to  upwards  of  1200  feet.  In  the  United  States,  beds  of 
rock-salt  are  known  to  occur  in  the  states  of  New  York,  Penn- 
sylvania, Ohio,  Virginia,  West  Virginia,  Michigan,  Kansas, 
Kentucky,  Texas,  Wyoming,  California,  and  Nevada.  Canada, 
England,  the  Carpathian  Mountains,  the  Austrian  and  Bavarian 
Alps,  West  Germany,  the  Vosges,  the  Jura,  Spain,  the  Pyrenees 
and  Celtiberian  mountains,  all  contain  important  beds.  With 
the  rock-salt  are  not  infrequently  associated  other  salts,  as  above 
noted.  In  the  celebrated  Stassfurth  deposits,  sixteen  different 
compounds  in  the  shape  of  chlorides  and  sulphates  of  sodium, 
potassium,  magnesium,  calcium,  and  iron  have  been  determined, 
many  of  them  in  sufficient  quantity  to  be  of  commercial  value. 

(7)  THE  HYDROCARBON  COMPOUNDS 

Under  this  head  are  included  a  series  of  hydrocarbon  com- 
pounds varying  in  physical  properties  from  solid  to  gaseous, 
and  in  color  from  coal-black  through  brown,  greenish,  red,  and 
yellow  to  colorless.  Unlike  the  other  members  of  the  hydro- 
carbon series  yet  to  be  described,  they  are  not  the  residual 
products  of  plant  decomposition  in  situ,  but  are  rather  distilla- 


THE  HYDROCARBON  COMPOUNDS  121 

tion  products  from  deeply  buried  organic  matter  of  both  animal 
and  vegetable  origin.  The  different  members  of  the  series 
differ  so  widely  in  their  properties  and  uses  that  each  must  be 
discussed  independently.  The  grouping  of  the  various  com- 
pounds as  given  below  is  open  to  many  objections  from  a  strictly 
scientific  standpoint,  but,  all  things  considered,  it  seems  best 
suited  for  our  present  purposes.1 

Gaseous     .....     Marsh  gas  (natural  gas) 
Fluidal      .....     Petroleum  (naphtha) 

..  ,  f  Pittasphalt  (maltha) 
Viscous  and  semi-solid  {  ^^  ^ 

c  Asphalt  (bitumen) 
Elastic  ...          .     .  •(  Elaterite 

BltummOUS    .....    <  Iwurtzilite 

f  Albertite 
Solid     ......  -I  Grahamite 

I  Uintaite 
f  Succinite 
Resinous  ...............  J  Copalite 

I  Ambrite 


Marsh  G-as  (Natural  Cras).  —  This  is  a  colorless  and  odor- 
less gas  arising  from  the  decomposition  of  organic  matter 
protected  from  the  oxidizing  influence  of  atmospheric  air.  By 
itself  it  burns  quietly  with  a  slightly  luminous  flame,  but  when 
mixed  with  air  forms  a  dangerous  explosive.  It  is  this  gas 
which  forms  the  dreaded  fire-damp  of  the  miners. 

Under  this  head  may  properly  be  considered  the  so-called 
natural  gas,  which  has  of  late  years  become  of  so  much  impor- 
tance from  an  economic  standpoint.  This  is,  however,  by  no 
means  a  simple  compound,  but  an  admixture  of  several  gases, 
samples  from  different  wells  showing  considerable  variation  in 
composition,  as  well  as  those  from  the  same  well  collected  at 
different  periods.  This  last  is  shown  by  the  six  analyses  fol- 
lowing, and  which  may  serve  well  to  illustrate  the  average 
composition,  though  in  some  instances  the  percentage  of  marsh 
gas  has  been  found  greater. 

1  W.  P.  Blake,  Trans.  Am.  Inst.  of  Mining  Engineers,  Vol.  XVIII,  1890, 
p.  582. 


122 


AQUEOUS  EOCKS 


CONSTITUENTS 

. 

II 

III 

IV 

v 

VI 

Mash  gas 

57.85% 

75.16  % 

72.18% 

65.25  % 

60.70% 

49.58% 

Hydrogen  

9.64 

14.45 

20.02 

26.16 

29.03 

35.92 

Ethylic  hydride  .... 
Olifiant  gas        .... 

5.20 
0.80 

4.80 
0.60 

3.60 
0.70 

5.50 

0.80 

7.92 
0.98 

12.30 
0.60 

Oxygen 

2.10 

1.20 

1.10 

0.80 

0.78 

0.80 

Carbonic  oxide  .... 
Carbonic  acid    .... 
Nitrogen  .          .... 

1.00 
0.00 
23.41 

0.30 
0.30 

2.89 

1.00 

0.80 
0.00 

0.80 
0.60 
0.00 

0.58 
0.00 
0.00 

0.40 
0.40 
0.00 

100.00  % 

99.70% 

99.40% 

99.91  % 

99.99  % 

100.00  % 

Natural  gas  in  quantities  sufficient  to  be  of  economic  impor- 
tance is  necessarily  limited  to  rocks  of  no  particular  horizon. 
The  tendency  of  recent  studies  seems  to  be  to  show  that  it 
results,  as  above  stated,  from  the  deeply  buried  organic  matter, 
of  both  plant  and  animal  origin.  It  is  not,  however,  indige- 
nous to  the  rocks  in  which  it  is  now  found,  but  occurs  in  an 
overlying,  more  or  less  porous,  sand  or  lime  rock  into  which 
it  has  been  forced  by  hydrostatic  pressure.  The  first  necessary 
condition  for  the  presence  of  gas  in  any  locality  may,  indeed, 
be  said  to  depend  upon  the  existence  of  such  a  porous  rock  as 
will  serve  as  a  reservoir  to  hold  it,  and  also  the  presence  of  an 
impervious  overlying  stratum  to  prevent  its  escape.  In  Penn- 
sylvania the  reservoir  rock  is  a  sandstone  of  Carboniferous  or 
Devonian  age ;  in  Ohio  and  Indiana,  a  cavernous  dolomitic 
limestone  of  Silurian  (Trenton)  age. 

Natural  gas,  as  may  readily  be  understood,  is  still  in  process 
of  formation,  though  at  a  rate  vastly  slower  than  it  is  being 
utilized,  or  wasted,  in  many  regions.  It  is  a  necessary  conse- 
quence that  the  available  supply  must  sooner  or  later  become 
exhausted.  Indeed  this  contingency  has  already  made  itself 
apparent  in  many  fields,  necessitating  continuous  activity  in 
prospecting,  and  in  more  than  one  instance  all  known  sources 
of  supply  are  already  exhausted.  Few  more  marked  illustra- 
tions of  man's  unreasonable  squandering  of  nature's  resources 
have  ever  been  offered  than  that  relating  to  the  utilization  of 
natural  gas. 

Petroleum.  —  This  is  the  name  given  to  a  complex  hydro- 
carbon compound,  liquid  at  ordinary  temperatures,  though 
varying  greatly  in  viscosity,  of  a  black,  brown,  greenish,  or 


THE  HYDROCARBON  COMPOUNDS 


123 


more  rarely,  red  or  yellow  color,  and  of  extremely  disagreeable 
odor.  Its  specific  gravity  varies  from  0.6  to  0.9.  Through 
becoming  more  and  more  viscous,  the  material  passes  into  the 
solid  and  semi-solid  forms,  asphalt  and  maltha.  Chemically  it 
is  considered  as  a  mixture  of  the  various  hydrocarbons  included 
in  the  marsh  gas,  ethyline,  and  paraffin  series. 

An  ultimate  analysis  of  several  samples,  as  given  by  the 
reports  of  the  10th  Census  of  the  United  States  (1880),  showed 
the  following  percentages  of  the  three  essential  constituents:  — 


LOCALITIES 

HYDROGEN 

CARBON 

NITROGEN 

West  Virginia 

13.359  % 

85.200  % 

0.540  % 

Mecca   Ohio 

13.071 

86  316 

0.230 

California    

11.819 

86.934 

1.109 

As  with  marsh  gas,  petroleum  is  considered  as  a  product  of 
organic  decomposition,  which  has  been  for  the  most  part  forced 
up  from  the  rocks  in  which  it  originated  into  overlying  strata. 
It  is  therefore  limited  to  no  particular  geological  horizon,  but 
is  found  in  rocks  of  all  ages,  from  the  Cambrian  to  the  most 
recent,  its  existence  in  quantities  sufficient  for  economic  pur- 
poses being  dependent  upon  local  conditions  for  its  generation 
and  subsequent  preservation.  Inasmuch  as  its  accumulation 
in  large  quantities  necessitates  a  rock  of  porous  nature  to  act  as 
a  reservoir,  the  petroleum-bearing  rocks  are  mostly  sandstones, 
though  not  uniformly  so.  Petroleums  are  found  in  California 
and  Texas,  in  Tertiary  sands ;  in  Colorado,  in  the  Cretaceous  ; 
in  West  Virginia,  both  above  and  below  the  Crinoidal  (Car- 
boniferous) limestones  ;  in  Pennsylvania,  in  the  Mountain  sands 
(Lower  Carboniferous)  and  the  Venango  sands  (Devonian);  in 
Canada,  in  the  Corniferous  (Lower  Devonian)  limestone  ;  in 
Kentucky,  in  the  Hudson  River  shales  (Lower  Silurian);  and 
in  Ohio,  in  the  Trenton  limestone,  also  of  Lower  Silurian  age. 

In  some  instances  petroleum  oozes  naturally  from  the  ground, 
forming  at  times  a  thin  layer  on  the  surface  of  pools  of  water, 
whence  in  times  past  it  has  been  gathered  and  used  for  chemical 
and  medicinal  purposes.  The  so-called  "  Seneca  oil "  thus  used 
some  fifty  or  sixty  years  ago  was  obtained  from  a  spring  in  Cuba, 
Alleghany  County,  in  New  York.  The  immense  supply  now 


124  AQUEOUS   ROCKS 

demanded  for  commercial  purposes  is,  however,  obtained  alto- 
gether from  artificial  wells  of  varying  depths,  and  which  are 
in  some  cases  self-flowing,  while  .in  others  the  oil  is  raised  by 
means  of  pumps.  Wells  of  from  500  to  1500  feet  in  depth  are 
of  common  occurrence,  while  those  upwards  of  2000  feet  are  not 
rare.  The  principal  sources  of  petroleum,  in  the  United  States, 
are  in  New  York,  Pennsylvania,  and  Ohio,  with  smaller  fields 
in  West  Virginia,  Kentucky,  Tennessee,  Indiana,  Texas,  Colo- 
rado, and  California.  The  chief  foreign  source  is  the  Baku 
region,  on  the  Caspian  Sea,  and  Galicia,  in  Austria. 

The  quantity  of  petroleum  and  semi-solid  bituminous  com- 
pounds contained  in  the  rocks  of  certain  areas  is  sometimes 
enormous.  Dr.  Hunt  estimated  that  the  dolomite  underlying 
the  city  of  Chicago  and  vicinity  contains  for  each  square  mile 
over  7,000,000  barrels.  A  like  computation  by  Professor 
Orton  1  led  to  the  conclusions  given  in  the  following  quotation 
relative  to  the  water-lime  stratum  of  Ohio,  which  is  almost 
universally  petroliferous:  — 

"Estimating  its  petroleum  contents  at  one-tenth  of  one  per 
cent,  and  the  thickness  of  the  stratum  at  500  feet,  both  of 
which  estimates  are  probably  within  the  limits,  we  find  the 
petroleum  contained  in  it  to  be  more  than  2,500,000  barrels  to 
the  square  mile.  The  total  production  of  the  great  oil  field 
of  Pennsylvania  and  New  York  to  January,  1885,  is  261,000,000 
barrels.  It  would  require  only  three  ordinary  townships,  or  a 
little  more  than  100  square  miles,  to  duplicate  this  enormous 
stock  from  the  water-lime  alone.  But  if  the  rate  of  one-tenth 
of  one  per  cent  should  be  maintained  through  a  descent  of 
1500  feet  at  any  point  in  the  state,  each  square  mile  would,  in 
that  case,  yield  75,000,000  barrels,  or  nearly  one-third  of  the 
total  product  of  the  entire  Pennsylvania  and  New  York  oil 
fields.  These  figures  pass  at  once  beyond  clear  comprehension, 
but  they  serve  to  give  some  idea  of  the  vast  stock  of  petroleum 
contained  in  the  earth's  crust.  If  petroleum  is  generally  dis- 
tributed through  a  considerable  series  of  rocks  in  any  appre- 
ciable percentage,  it  is  easy  to  see  that  the  aggregate  amount 
must  be  immense.  Even  one-thousandth  of  one  per  cent  would 
yield  750,000  barrels  to  the  square  mile  in  a  series  of  rocks  1500 
feet  deep,  but  this  amount  is  nearly  equal  to  the  greatest  actual 
production  per  square  mile  of  any  part  of  the  leading  Pennsyl- 

1  Ann.  Rep.  U.  S.  Geol.  Survey,  1886-87,  Part  II,  p.  507. 


THE  HYDROCARBON  COMPOUNDS  125 

vania  fields.  It  is  obvious  that  the  total  amount  of  petroleum 
in  the  rocks  underlying  the  surface  of  Ohio  is  large  beyond 
computation,  but  in  its  diffused  and  distributed  state  it  is 
entirely  without  value.  It  must  be  accumulated  in  rocks  that 
serve  as  reservoirs  before  it  becomes  of  economic  interest.  In 
respect  to  the  importance  of  concentration,  it  agrees  with  most 
other  forms  of  mineral  wealth." 

Asphaltum  (JBitumen,  or  Mineral  Pitch).  — These  are  names 
given  to  what  are  rather  indefinite  admixtures  of  various 
hydrocarbons,  in  part  oxygenated,  and  which,  for  the  most  part 
solid  or  at  least  highly  viscous  at  ordinary  temperatures,  pass 
by  insensible  gradations  into  pittasphalts  or  mineral  tar,  and 
these  in  turn  into  the  petroleums.  They  are  characterized  by 
a  black  or  brownish  black  color,  pitchy  lustre,  and  bituminous 
odor.  The  solid  forms  melt  ordinarily  at  a  temperature  of 
from  90°  to  100°  F.,  and  burn  readily  with  a  bright  flame, 
giving  off  dense  fumes  of  a  tarry  odor.  The  fluidal  varieties 
become  solid  on  exposure  to  the  atmosphere,  owing  to  evapora- 
tion of  the  more  volatile  portions. 

The  crude  asphalt  of  Trinidad  has  the  following  composition 
and  physical  characteristics  : 1  — 

Specific  gravity,  1.28  ;  hardness  at  70°  F.,  2.5  to  3,  Dana's 
scale  ;  color,  chocolate-brown.  Composition  :  — 

Bitumen 39.83  % 

Earthy  matter 33.99 

Vegetable  matter      ....      9.31 

Water  16.87 


100.00% 

The  mode  of  occurrence  of  asphalt  deposits  varies  greatly, 
owing  to  the  fact  that,  as  with  petroleum  and  natural  gas,  it 
has  come  up  through  fissures  and  cracks  in  the  earth's  surface, 
and  as  a  rule  no  longer  occupies  its  place  of  origin.  On  the 
island  of  Trinidad  is  an  immense  superficial  deposit  having  an 
area  of  about  114  acres  and  a  depth  varying  from  18  to  78  feet. 
The  surface  is  sufficiently  solid  over  nearly  every  part  for  the 
passage  of  teams,  is  of  a  brownish  black  color,  and  nearly  level. 
The  deposit  has  in  numerous  publications  been  compared  to  a 
lake,  and  stated  to  be  fluidal  and  at  a  high  temperature  in  the 
centre.  This  statement  is  quite  erroneous  and  misleading. 

1  Trans.  Am.  Inst.  Mining  Engineers,  Vol.  XVII,  1889,  p.  363. 


126  AQUEOUS   ROCKS 

In  Ventura  County,  California,  the  material  occurs  in  a  fissure 
vein  in  siliceous  clay  of  Miocene  age,  the  vein  being  from  7  to 
15  inches  thick  on  the  surface,  but  widening  rapidly  in  descent 
to  a  thickness  of  5  feet  at  a  depth  of  65  feet  below  the  surface. 
The  material  of  the  vein  is,  however,  far  from  pure  asphalt ; 
but  rather  an  asphaltic  sand.  In  western  Kentucky  the  as- 
phalt exudes  from  the  ground  in  the  form  of  utar  springs,"  and 
occurs  also  disseminated  through  sandstones  and  limestones  of 
sub-Carboniferous  age.  Frequently,  as  in  the  dolomite  under- 
lying Chicago,  Illinois,  the  bituminous  matter  is  so  diffused 
throughout  the  rock  as  to  give  it,  on  exposure,  a  brownish 
black  appearance,  and  cause  it  to  exhale  an  odor  of  petroleum 
appreciable  for  some  distance.  In  the  Dead  Sea,  bituminous 
masses  of  considerable  size  have  in  times  past  risen  like  islands 
to  the  surface  of  the  water,  and  furnished  thus  the  material 
used  by  the  ancients  in  pitching  the  walls  of  buildings  and 
rendering  vessels  water-tight.  The  ancient  name  of  this  body 
of  water  was  Lake  Asphaltites,  and  from  it  our  word  asphalt 
is  derived. 

The  above  illustrations  are  sufficient  to  indicate  the  numerous 
conditions  under  which  the  substance  occurs.  The  material  is 
world-wide  in  its  geographic  distribution  and  equally  cosmo- 
politan in  its  geological  range,  being  found  in  gneissic  rocks  of 
presumably  Archaean  age  in  Sweden,  and  in  rocks  of  all  inter- 
mediate horizons  down  to  late  Tertiary. 

Elaterite  (Mineral  Caoutchouc).  —  This  is  the  name  given  to  a 
soft  and  elastic  variety  of  asphalt  much  resembling  pure  india- 
rubber.  It  is  easily  compressible  in  the  fingers,  to  which  it 
adheres  slightly,  of  a  brownish  color,  and  of  a  specific  gravity 
varying  from  0.905  to  1.00.  It  has  been  described  from  mines 
in  Derbyshire  and  elsewhere  in  England,  but,  so  far  as  the 
writer  is  aware,  is  of  no  commercial  value.  Its  composition  so 
far  as  determined  is,  carbon,  85.47  %;  hydrogen,  13.28  %. 

The  name  wurtzilite  has  been  given  by  Professor  W.  P. 
Blake  to  a  hydrocarbon  very  similar  in  appearance  to  the 
uintaite  (described  below),  but  differing  in  physical  and  chem-- 
ical  properties.  It  is  described  as  a  firm  black  solid,  amorphous 
in  structure,  brittle  when  cold,  breaking  with  a  conchoidal 
fracture,  but  when  warm,  tough  and  elastic,  its  elasticity  being 
best  compared  with  that  of  mica.  If  bent  too  quickly,  it  snaps 
like  glass.  It  cuts  like  horn,  has  a  hardness  between  2  and  3, 


THE  HYDROCARBON  COMPOUNDS  127 

a  specific  gravity  of  1.03,  gives  a  brown  streak,  and  in  very 
thin  flakes  shows  a  garnet-red  color.  It  does  not  fuse  or 
melt  in  boiling  water,  bnt  becomes  softer  and  more  elastic ;  in 
the  flame  of  a  candle  it  melts  and  takes  fire,  burning  with  a 
bright,  luminous  flame,  giving  off  gas  and  a  strong  bitumi- 
nous odor.  It  is  not  soluble  in  alcohol,  but  sparingly  so 
in  ether,  in  both  of  which  respects  it  differs  from  elaterite 
proper. 

Albertite.  —  This  is  a  brilliant  jet-black  compound,  breaking 
with  a  lustrous,  conchoidal  fracture,  having  a  hardness  of 
between  1  and  2  of  Dana's  scale,  a  specific  gravity  of  1.097, 
a  black  streak,  and  showing  a  brown  color  on  very  thin  edges. 
In  the  flame  of  a  lamp  it  shows  signs  of  incipient  fusion,  intu- 
mesces  somewhat,  and  emits  jets  of  gas,  giving  off  a  bituminous 
odor ;  when  rubbed  it  becomes  electric.  According  to  Dana, 
it  softens  slightly  in  boiling  water,  is  scarcely  at  all  soluble  in 
alcohol,  and  only  slightly  so  in  ether  and  in  turpentine.  The 
following  is  the  composition  as  given  by  Witherill :  Carbon, 
86.04%;  hydrogen,  8.96%;  oxygen,  1.97%;  nitrogen,  2.93%; 
ash,  0.10%.  The  mineral  occurs  in  fissures  in  rocks  of  sub- 
Carboiiiferous  age,  at  the  Albert  Mines,  in  Hillsborough  County, 
Nova  Scotia ;  hence  the  name. 

Formerly  it  was  used  for  the  distillation  of  oils  for  illumi- 
nating purposes.  Since  the  discovery  of  petroleum  its  use  has 
been  discontinued. 

G-rahamite. — This  variety  resembles  the  last  in  its  general 
appearance  and  its  conduct  toward  solvents,  and  it  is  a  question 
if  it  is  not  identical  therewith.  It  was  described  by  Dr.  Wurtz 
from  Ritchie  County,  in  West  Virginia,  where  it  occurred  in  a 
vein  some  four  feet  in  width  in  Carboniferous  sandstones. 

Uintaite  (Grilsonite).  — This  is  a  black,  brilliant,  and  lustrous 
variety  giving  a  dark-brown  streak,  breaking  with  a  beautiful 
conchoidal  fracture,  and  having  a  hardness  of  2  to  2.5  and  a 
specific  gravity  of  1.065  to  1.07.  It  fuses  readily  in  the  flame 
of  a  candle,  is  plastic  but  not  sticky  while  warm,  and  unless 
highly  heated  will  not  adhere  to  cold  paper.  Its  deportment 
is  stated  to  be  much  like  that  of  sealing  wax  or  shellac.  Like 
albertite  and  grahamite,  it  dissolves  slightly  in  turpentine  and 
is  not  soluble  in  alcohol.  It  is  a  good  non-conductor  of  elec- 
tricity, but  like  albertite  becomes  electric  by  friction.  Its 
composition  as  given  is,  carbon,  80.88%;  hydrogen,  9.76%-, 


128  AQUEOUS   ROCKS 

nitrogen,  3.30  %;  oxygen,  6.05  %  ;  and  ash,  0.01  %.  The  min- 
eral as  first  described  occurred  in  a  vertical  vein  from  3  to  5 
feet  in  thickness,  cutting  through  nearly  horizontal  sandstones 
some  3  miles  east  of  Fort  Duchesne,  on  the  reservation  of  the 
Uinta  Indians. 

/Succinite  (Am~ber).  —  The  mineral  commonly  known  as  amber 
is  a  fossil  resin,  consisting  of  some  78.94  parts  of  carbon,  10.53 
parts  of  oxygen,  and  10.53  parts  of  hydrogen,  together  with 
usually  from  two  to  four-tenths  of  a  per  cent  of  sulphur.  It 
is  not  a  simple  resin,  but  a  compound  of  four  or  more  hydro- 
carbons. According  to  Berzelius,  as  quoted  by  Dana,  it  "  con- 
sists mainly  of  (85%  to  '90%)  two  other  resins  in  soluble 
alcohol  and  ether,  and  an  oil,  and  2J-  %  to  6  %  of  succinic 
acid." 

The  mineral,  as  found,  is  of  a  yellow,  brownish,  or  reddish 
color,  frequently  clouded,  translucent,  or  even  transparent, 
tasteless,  becomes  negatively  electrified  by  friction,  has  a  hard- 
ness of  2  to  2.5,  a  specific  gravity,  when  free  from  enclosures,  of 
1.096,  a  conchoidal  fracture,  and  melts  at  250°  to  300°  Fahr. 
without  previous  swelling,  but  boils  quietly,  giving  off  dense 
white  fumes  with  an  aromatic  odor  and  very  irritating  effect 
on  the  respiratory  organs. 

Amber,  or  closely  related  compounds,  has  been  found  in 
varying  amounts  at  numerous  widely  separated  localities,  but 
always  under  conditions  closely  resembling  one  another.  The 
better  known  localities  are  the  Prussian  coast  of  the  Baltic ;  on 
the  coast  of  Norfolk,  Essex,  and  Suffolk,  England  ;  the  coasts 
of  Sweden,  Denmark,  and  the  Russian  Baltic  provinces;  in 
Galicia,  Westphalia  ;  Poland  ;  Moravia  ;  in  Norway  ;  Switzer- 
land ;  France ;  Upper  Burma ;  Sicily ;  Mexico ;  the  United 
States  at  Martha's  Vineyard,  and  near  Trenton  and  Camden, 
New  Jersey,  and  at  Cedar  Lake  in  Northwest  Canada. 

The  amber  of  commerce  comes  now,  as  for  the  past  2000 
years,  mainly  from  the  Baltic,  where  it  occurs  in  a  stratum  of 
blue  earth  of  from  4  to  20  feet  in  thickness  underlying  the 
brown  coal  formation. 

Ozokerite  (Mineral  Wax;  Native  Paraffin).  —  This  is  a  wax- 
like  hydrocarbon,  usually  with  a  foliated  structure,  soft  and 
easily  indented  with  the  thumb  nail ;  of  a  yellow,  yellow 
brown,  or  sometimes  greenish  color,  translucent  when  pure, 
with  a  greasy  feeling,  and  fusing  at  56°  to  63°  F. ;  specific 


ROCKS   FORMED   AS    SEDIMENTARY   DEPOSITS  129 

gravity,  0.955.  It  is  essentially  a  natural  paraffin.  The  name 
is  derived  from  two  Greek  words,  signifying  to  smell,  and  ivax. 
Below  is  given  the  composition  of  (I)  samples  from  Utah,  and 
(II)  from  Boryslaw,  in  Galicia. 


CONSTITUENTS 

I 

II 

Carbon      

85.47% 

85  78  % 

Hydrogen 

14  57 

14  ">9 

100.04% 

100.07  % 

The  substance  is  completely  soluble  in  boiling  ether,  carbon 
disulphide,  or  benzine,  and  partially  so  in  alcohol. 

Ozokerite  occurs  in  the  United  States,  in  Emery  and  Uinta 
counties,  Utah,  where  in  the  form  of  small  veins  in  Tertiary 
rocks  it  extends  over  a  wide  area.  It  is  also  found  in  Galicia, 
Austria,  in  Miocene  deposits,  in  Roumania,  Hungary,  Russia, 
and  other  Asiatic  and  European  sources.  As  a  rule  the  de- 
posits are  in  beds  of  Tertiary  or  Cretaceous  age.  The  Galician 
deposits  are  the  most  noted  of  the  above.  According  to 
Boverton  Redwood,1  the  material  occurs  here  in  the  form  of 
veins  from  the  thickness  of  a  few  millimetres  to  some  feet,  and 
is  accompanied  by  petroleum  and  gaseous  hydrocarbons. 

The  names  scheerite,  hatchettite,  ficlitellite,  and  konlite  are 
applied  to  simple  hydrocarbons  allied  to  ozokerite  found  in 
beds  of  peat  and  coal,  but  so  far  as  the  writer  is  aware  never  in 
such  abundance  as  to  be  of  commercial  value. 

The  name  retinite  includes  a  considerable  series  of  fossil 
resins  allied  to  amber,  differing  mainly  in  containing  no  suc- 
cinic  acid.  They  occur  in  beds  of  brown  coal  of  Tertiary  and 
Cretaceous  age.  The  so-called  copalite,  a  hard  brittle,  clear 
yellow,  or  brownish  variety  used  in  making  varnishes,  belongs 
here. 

2.     ROCKS  FORMED  AS  SEDIMENTARY  DEPOSITS  AND  FRAG- 
MENTAL  IN  STRUCTURE:   CLASTIC 

The  rocks  of  this  group  differ  from  those  just  described  in 
that  they  are  composed  mainly  of  fragmental  materials  derived 
from  the  breaking  down  of  older  rocks,  or  are  but  the  more  or 

1  Jour.  Soc.  of  Chem.  Industry,  February,  1892. 


130  AQUEOUS   ROCKS 

less  consolidated  accumulations  of  organic  and  inorganic  debris 
from  plant  and  animal  life.  The  group  shows  transitional 
forms  into  the  last,  as  will  be  illustrated  by  certain  of  the  lime- 
stones and  the  quarzites.  They  are  water  deposits,  and,  as  a 
rule,  are  eminently  stratified  or  bedded,  although  this  structure 
is  not  always  apparent  in  the  hand  specimen. 

As  will  be  readily  comprehended  when  one  considers  from 
what  a  multitude  of  materials  the  fragmental  rocks  have  been 
derived,  the  amount  of  assorting,  admixture  with  other  sub- 
stances, solution,  and  transportation  by  streams  these  materials 
have  undergone,  they  cannot  be  classified  by  any  hard  and  fast 
lines,  but  one  variety  may  grade  into  another,  both  in  texture 
and  structure  as  well  as  in  chemical  composition,  almost  indefi- 
nitely. Indeed,  many  of  them  can  scarcely  be  considered  as 
more  than  indurated  muds,  and  only  very  general  names  can 
be  given  them. 

Accordingly  as  these  rocks  consist  of  mechanically  formed 
inorganic  particles  of  varying  composition  and  texture,  or  of 
the  more  or  less  fragmental  debris  from  plant  and  animal  life, 
they  are  here  divided  into  two  main  groups,  each  of  which  is 
subdivided  as  below  :  — 

I.  Rocks  formed  by  mechanical  agencies,  and  mainly  of  in- 
organic materials. 

(1)  The  Arenaceous  group  —  Psammites:  Sand,  gravel,  sand- 
stone, conglomerate,  and  breccia. 

(2)  The  Argillaceous  group  —  Pelites  :  Kaolin,  clay,  wacke, 
shale,  clayey  marl,  argillite. 

(3)  The    Calcareous   group  :  —  Arenaceous   and   brecciated 
limestones.     The  rocks  of  this  group  are  often  in  part  organic, 
and  in   part   chemical    deposits.     Only   those   are   considered 
here  in  which  the  fragmental  nature  is  the  most  pronounced 
characteristic. 

(4)  The   Volcanic    group  :  —  Fragmental    rocks    composed 
mainly  of  ejected  volcanic  material  :  Tuffs,  lapilli,  sand  and 
ashes,  pumice-dust,  trass,  peperino,  pozzuolano,  etc. 

•  II.  Rocks  formed  largely  or  only  in  part  by  mechanical 
agencies  and  composed  mainly  of  the  debris  from  plant  and 
animal  life  —  Organagenous. 

(1)  The  Siliceous  group  —  Infusorial  earth. 

(2)  The  Calcareous  group  —  Fossiliferous  and  oolitic  lime- 
stone, marl,  shell-sand,  shell-rock. 


PLATE    11 


FIGS.  1  and  2.   Shell  limestones.  FIG.  3.   Crinoldal  limestone. 


ARENACEOUS   ROCKS:    PSAMMITES 


131 


(3)  The  Carbonaceous  group  —  Peat,  lignite,  coals,  oil  shale, 
etc. 

(4)  The   Phosphatic  group  —  Phosphatic  sandstone,  guano, 
coprolite  nodules. 


(1)    ROCKS   COMPOSED   MAINLY   OF   INORGANIC   MATERIAL 

(1)  The  Arenaceous  Group:  Psammites. — Arenaceous,  from 
the  Latin  arenaceous,  sandy  or  sand-like  ;  psammite  from  the 
Greek  -v/ra/LtyLuVr;?,  sandy. 

These  rocks  are  composed  mainly  of  the  siliceous  materials 
derived  from  the  disintegration  of  older  crystalline  rocks  and 
which  have  been  rearranged  in  beds  of  varying  thickness 
through  the  mechanical  agency  of  water.  They  are,  in  short, 
more  or  less  consolidated  beds  of  sand  and  gravel.  In  composi- 
tion and  texture,  they  vary  almost  indefinitely.  Many  of  them 
having  suffered  little  during  the  process  of  disintegration  and 
transportation,  are  com- 
posed of  essentially  the 
same  materials  as  the 
rocks  from  which  they 
were  derived.  Others, 
in  which  the  fragmeiital 
materials  suffered  more 
prior  to  their  final  con- 
solidation, have  had  the 
softer  and  more  soluble 
minerals  removed,  leav- 
ing the  sand  composed 
mainly  of  the  hard,  al- 
most indestructible  min- 
eral quartz. 

In  structure,  the  sand- 
stones also  vary  greatly, 
in  some  the  grains  being  rounded,  while  in  others  they  are 
sharply  angular.  Figure  9  shows  the  microscopic  structure  of 
a  brown  Triassic  sandstone  from  Portland,  Connecticut. 

The  material  by  which  the  individual  grains  of  a  sandstone 
are  bound  together  is  as  a  rule  of  a  calcareous,  ferruginous,  or 
siliceous  nature ;  sometimes  argillaceous.  The  substance  has 
been  deposited  between  the  granules  by  percolating  water  or 


FIG.  9.  —  Microstructure  of  sandstone, 
Portland,  Connecticut. 


132 


AQUEOUS   ROCKS 


during  the  process  of  sedimentation,  and  forms  a  natural 
cement.  It  sometimes  happens  that  the  siliceous  cement  is 
deposited  about  the  rounded  grains  of  quartz  in  the  form  of  a 
new  crystalline  growth,  converting  the  stone  into  quartzite  ; 
such  are  in  this  work  classed  with  the  crystalline  rocks. 

Upon  the  character  of  this  cementing  material  and  the  close- 
ness with  which  the  grains  are  bound  together,  is  very  largely 
dependent  the  power  of  the  stone  to  resist  disintegration  under 
the  trying  action  of  percolating  carbonated  waters  and  the 
mechanical  action  of  heat  and  frost.  The  calcareous,  and  to  a 
less  extent  the  ferruginous  cements  are  liable  to  removal  in 
solution,  allowing  the  rock  to  fall  away  to  sand,  or  at  least 
allowing  it  to  absorb  water,  which,  on  freezing,  brings  about 
the  disintegration.  The  argillaceous  cementing  material,  while 
in  itself  inert,  also  permits  a  high  degree  of  absorption,  with 
like  results.  Those  sandstones  cemented  by  silica,  and  which 
therefore  partake  of  the  nature  of  quartzite  (see  p.  169),  are 
by  far  the  more  refractory. 

The  following  analyses  will  serve  to  indicate  the  consid- 
erable range  in  composition  of  rocks  of  this  class  :  — 


CONSTITUENTS 

I 

II 

III 

IV 

Silica  (Si02)    

69  94  % 

84.40  % 

95.24% 

90.86% 

Alumina  (Al20s)      

13.15 

7.49 

0.56 

4.76 

Iron  oxides  (Fe203)  and  (FeO)     .     . 
Manganese  (MnO) 

2.48 
0  70 

3.87 

1.28 

1.58 

Lime  (CaO)    . 

3.09 

0.74 

1.40 

0.15 

Magnesia  (MgO)      

Trace 

2.11 

1.23 

0.59 

Potash  (K20)  

3.30 

0.24 

1.06 

Soda  (Na20)   
Loss  

5.43 
1  01 

0.56 

0.56 

0.45 

Totals  

99.  10  % 

99  41  % 

99.27  % 

99.45% 

I.  Brown  Triassic  sandstone:  Portland,  Connecticut.  II.  Gray  sub-Carbo- 
niferous sandstone:  Berea,  Ohio.  III.  Red  Carboniferous  sandstone:  Anan, 
Scotland.  IV.  Cambrian  sandstone  :  Siskowit  Bay,  Wisconsin. 

The  table  given  on  p.  166  will  serve  to  show  the  close  chemi- 
cal relationship  existing  between  many  rocks  of  this  group, 
and  their  metamorphic  equivalents. 

The  colors  of  sandstone  are  dependent  upon  a  variety  of 
circumstances.  The  red,  brown,  and  yellowish  colors  are  due 


ARENACEOUS   ROCKS:   PSAMMITES  133 

to  iron  oxides  in  the  cementing  constituent.  Some  of  the  dark 
colors  are  due  to  carbonaceous  matter. 

Many  varieties  of  sandstone  are  popularly  recognized.  Cal- 
careous, ferruginous,  siliceous,  or  argillaceous  sandstones  are  those 
in  which  the  cementing  materials  are  of  a  calcareous,  ferrugi- 
nous, siliceous,  or  argillaceous  nature.  The  name  arkose  is  given 
to  a  coarse  feldspathic  sandstone  derived  from  granitic  rocks, 
with  a  minimum  amount  of  loss  of  original  material.  Conglomer- 
ate or  puddingstone  is  merely  a  coarse  sandstone  ;  it  differs  from 
ordinary  sandstone  only  as  gravel  differs  from  sand.  Breccia 
is  a  fragmental  rock  differing  from  conglomerate  in  that  the 
individual  particles  are  sharply  angular  instead  of  rounded. 
The  term  is  made  to  include  also  certain  volcanic  rocks  with  a 
brecciated  structure.  (See  PI.  4.) 

Crreywacke  or  Grramvacke  is  an  old  German  name  for  brecci- 
ated fragmental  rocks  made  up  of  argillaceous  particles.  The 
name  is  now  little  used.  Other  names,  &s  flagstone,  freestone,  and 
brownstone,  are  applied  to  such  as  are  used  for  flagging  or  other 
structural  purposes.  Itacolumite  is  a  feldspathic  sandstone,  or 
perhaps  more  properly  quartzite,  in  which  the  feldspathic  mate- 
rial plays  the  role  of  a  binding  constituent  to  the  quartz  gran- 
ules. The  so-called  flexible  sandstone  is  an  itacolumite  from 
which  the  feldspathic  portions  have  been  removed  by  decompo- 
sition leaving  the  interlocking  quartz  grains  with  a  small  amount 
of  play  between  them.  The  rock  is  in  no  sense  elastic,  but 
merely  loose  jointed. 

The  name  greensand,  greensand  marl,  and  glauconitic  sand  are 
given  to  a  prevailing  dull  green,  loosely  coherent,  clayey  or 
arenaceous  deposit  which  owes  its  peculiarities  to  the  presence 
of  the  hydrous  silicate  of  iron  and  potassium  glauconite,  but 
which  is  variously  contaminated  with  minute  particles  of  quartz 
and  siliceous  minerals  such  as  feldspar,  hornblende,  augite, 
garnet,  epidote,  tourmaline,  zircon,  and  the  iron  ores,  clay,  rock 
fragments,  and  particles  of  shells. 

Beds  of  glauconitic  sand  are  most  abundant  among  terranes 
of  Cretaceous  age,  but  are  by  no  means  limited  to  them,  as  has 
been  already  intimated  on  p.  31.  They  are  aqueous  deposits, 
formed  during  processes  of  slow  sedimentation  along  coasts 
receiving  debris  from  the  continental  slopes  and  of  a  nature 
such  as  is  derived  from  the  breaking  down  of  granitic  and  other 
feldspathic  rocks.  The  depth  at  which  such  deposits  form  is 


134 


AQUEOUS   ROCKS 


naturally  quite  variable,  but  conditions  most  favorable  to  their 
accumulation  seem  to  lie  just  beyond  the  reach  of  wave  agita- 
tion and  under  a  depth  of  900  fathoms. 

The  following  table  of  analyses  of  glauconitic  marls  is  from 
the  Report  of  the  Geological  Survey  of  New  Jersey,  for  1893. 


CONSTITUENTS 

I 

II 

III 

IV 

VI 

X 

XI 

XII 

XIII 

Phosphoric  acid  .  . 

l.°15 

0.58 

1.51 

1.14 

0.84 

0.19 

0.50 

6.87 

3.73 

Sulphuric  acid  .  .  . 

1.28 

.... 

2.40 

0.14 

0.12 

0.41 

0.34 

3.12 

2.44 

Silica  and  sand  .  .  . 

34.50 

45.50 

55.69 

38.70 

52.07 

51.15 

47.50 

44.68 

49.68 

Potash               .  .  . 

1.54 

3.79 

5.27 

3.65 

6.46 

7.08 

5.29 

397 

4.98 

Lime  

2.52 

1.51 

0.65 

9.07 

1.01 

0.49 

0.56 

4.97 

4.14 

Magnesia  

2.15 

2.20 

0.79 

1.50 

1.53 

2.02 

2.70 

2.97 

0.47 

Alumina 

6.00 

5.80 

6.61 

10.20 

6.96 

8  23 

860 

604 

f> 

Oxide  of  iron  .... 

31.50 

24.50 

21.63 

18.63 

21.55 

23.13 

20.52 

18.97 

28.71 

Water  .        ... 

18.80 

15.40 

8.85 

10.00 

9.31 

6.67 

1357 

863 

554 

Carbonic  acid    .  .  . 

6.14 

Carbonate  of  lime 

99.43 

99.18 

102.40 

99.16 

99.85 

99.37 

99.58 

99.32 

99.69 

I.  Clay  marl,  from  near  Mattawan.  II.  Clay  marl,  from  Matchaponix 
Creek,  three  miles  south  of  Spottswood.  III.  Lower  marl,  from  Navesink 
Highlands.  IV.  Lower  marl,  from  north  shore  of  Navesink  River,  at  Red  Bank. 
VI.  Lower  marl,  from  northwest  slope  of  Mount  Pleasant  Hills.  X.  Middle 
marl,  from  near  Eatontown.  XI.  Middle  marl,  from  southeast  of  Freehold. 
XII.  Upper  marl,  from  Poplar.  XIII.  Upper  marl,  from  Shark  River. 

The  most  extensive  and  best  known  deposits  in  the  United 
States  are  included  in  what  are  known  as  the  Upper,  Middle, 
and  Lower  marl  beds  of  the  Cretaceous  formations  in  south- 
eastern New  Jersey,  and  which  has  been  very  thoroughly 
described  in  the  various  reports  of  the  State  Survey.1  The 
marl  is  somewhat  variable  in  different  localities,  but  may  in  a 
general  way  be  described  as  a  dull  green,  arenaceous  deposit  of 
such  consistency  as  to  be  easily  removed  by  the  shovel  alone, 
or  pick  and  shovel.  The  beds  vary  from  30  to  60  feet  in  thick- 
ness, but  the  glauconitic  layers  are  not  uniformly  distributed 
through  it.  Through  weathering,  the  ferruginous  constituents 
become  more  highly  oxidized,  and  the  color  changed  from  dull 
green  to  red  and  yellow. 

1  The  reader  is  especially  referred  to  Professor  W.  B.  Clarke's  paper  on  "  The 
Cretaceous  and  Tertiary  Formations  of  New  Jersey,"  in  the  Ann.  Rep.  State 
Geologist  of  New  Jersey  for  1892. 


ARGILLACEOUS   ROCKS:    PELITES  135 

Rocks  belonging  to  the  arenaceous  group  are  world  wide 
in  their  distribution,  covering  not  infrequently  thousands  of 
square  miles  of  territory  to  depths,  it  may  be,  of  thousands  of 
feet.  They  are,  in  some  of  their  varieties,  among  the  most 
common  and  wide-spread  of  materials.  Being  themselves  the 
products  of  disintegration  and  decomposition  of  pre-existing 
rocks,  and  having  become  consolidated  under  conditions  not 
greatly  different  from  those  now  existing  at  or  near  the  surface 
of  the  earth,  the  rocks  of  this  group  are  as  a  whole  in  a  state  of 
comparatively  stable  chemical  equilibrium.  Unless  including 
calcareous  matter  or  readily  oxidizing  ferruginous  compounds, 
such  are  subject  to  disintegration  more  through  physical  than 
chemical  agencies,  as  will  be  noted  later. 

(2)  The  Argillaceous  Group:  Pelites. — The  rocks  of  this 
group  are  composed  of  more  or  less  hydrated  aluminous  sili- 
cates admixed  in  almost  indefinite  proportions  with  siliceous 
sand,  various  silicate  minerals  in  a  more  or  less  fragmental  and 
decomposed  condition,  and  calcareous  and  carbonaceous  matter. 
In  their  least  consolidated  form  they  are  best  represented  by 
the  common  plastic  clays  used  for  brick  and  pottery  manufac- 
ture. Such,  although  alike  in  their  general  physical  or  even 
ultimate  chemical  nature,  have  widely  diverse  origins.  In  fact, 
the  term  clay,  like  silt,  indicates  physical  condition  rather  than 
chemical  or  miiieralogical  composition,  and  it  may  perhaps  be 
defined  as  an  indefinite  admixture  of  more  or  less  hydrated 
aluminous  silicates,  free  silica,  iron  oxides,  carbonates  of  lime, 
and  various  silicate  minerals  which  in  a  more  or  less  decom- 
posed and  fragmental  condition  have  survived  the  destructive 
agencies  to  which  they  have  been  subjected.  About  the  only 
feature  characteristic  of  all  clays,  is  that  of  plasticity,  when 
wet,  and  this  is  dependent,  apparently,  wholly  upon  texture 
and  structure,  i.e.  upon  the  size  and  shape  of  the  individual 
particles.  Pure  quartz,  chalcedony,  flint,  feldspar,  or  other 
silicates,  will,  when  reduced  to  an  impalpable  powder,  possess 
the  plasticity  and  even  odor  usually  ascribed  to  clay,  and  in  the 
pages  following,  the  term  is  used  only  with  reference  to  degree 
of  comminution,  regardless  of  mineral  nature  or  chemical  com- 
position. It  includes  residual  products  of  any  or  all  forms  of 
rock  degeneration,  and  which  may  or  may  not  have  been  re- 
assorted  through  the  agency  of  water.  (See  further  under  The 
Regolith,  Part  V.)  The  oft-repeated  statement  that  kaolin 


136 


AQUEOUS   ROCKS 


forms  the  basis  of  clays,  or  that  clay  is  impure  kaolin,  is  there- 
fore to  a  certain  extent  misleading,  and  if  accepted  at  all  it 
must  be  with  the  reservations  made  by  Johnson  and  Blake,1 
who  limit  the  term  kaolin  itself  to  the  impure  material,  quite 
distinct  from  true  kaolinite,  which  is  a  definite  chemical  com- 
pound corresponding  to  the  formula  H4Al2Si2O9. 

Throughout  the  glaciated  region  of  the  northeastern  United 
States  the  clays  are  mostly  glacial  or  water  deposits  from  the 
floods  of  the  Champlain  epoch.  The  latter  are  often  beauti- 
fully and  evenly  stratified,  as  shown  in  the  illustration  on  PI.  24. 
The  plastic  clays  and  siliceous  sands  about  Woodbridge,  New 
Jersey,  are  regarded  as  derived  from  the  Azoic  rocks  and 
deposited  by  sea- water  in  enclosed  basins.  The  exact  source  of 
the  material  is  not  always  apparent ;  the  porcelain  clays  of  Law- 
rence County,  Indiana,  on  the  other  hand,  are  residual  deposits 
resulting  from  the  decomposition  of  impure  Carboniferous 
(Archimides)  limestones,  the  lime  carbonate  being  removed  in 
solution,  while  the  less  soluble  clay  remains.  Kaolin,  as  already 
noted,  is  a  residual  deposit  from  the  decay  of  feldspathic  and 
other  aluminous  rocks,  while  the  ordinary  brick  and  tile  clays 
of  the  Southern  states,  as  well  as  the  clayey  soils,  are  residual 
aluminous  deposits  resulting  from  the  decay  and  leaching  out 
of  soluble  constituents  from  a  variety  of  rocks,  both  sedimentary 
and  eruptive.  (See  chapter  on  rock  weathering.) 

As  showing  the  comparative  compositions  of  kaolins  and 
clays,  the  following  table  is  given  :  — 


CONSTITUENTS 

I 

II 

III 

IV 

V 

VI 

SI02  (combined)      .    .    . 
Si02  (free)      

46.4  % 

39.00% 

34.70% 
12.20 

28.30% 

27.80 

42.71  % 
0.70 

J60.97% 

A1203      

39.7 

36.00 

31.34 

27.42 

39.24 

26.38 

H20  (combined)      .    .    . 
H20  at  212°     ... 

13.9 

14.00 
950 

12.00 

8  00 

6.60 

2  90 

13.32 
1  58 

}  8.93 

CaOandMgO      .... 
Alkalies  . 

.... 

0.63 
0  54 

0.10 
0  95 

0.18 
2  71 

0.20 
0  89 

}  1.90 

Fe203      .... 

0  16 

2  68 

0  46 

146 

»  •  *  • 

99.00  % 

99.67% 

99.45% 

98.59% 

99.10% 

99.64% 

I.  Kaolin.     II.  Indianite,  a  white  clay  residual  from  St.  Lawrence  County, 
Indiana.     III.  Potter's  clay,  from  Pope  County,  Illinois.     IV.  Brick  clay  from 
New  Jersey.     V.  Fire  clay  from  New  Jersey.     VI.  Fire  clay  from  Illinois. 
1  Am.  Jour,  of  Science,  1867,  p.  351. 


ARGILLITES  AND   SHALES 


137 


Amongst  the  older  formations  the  clays  have  undergone 
induration,  giving  rise  to  what  are  known  as  argillites,  or  if 
fissile,  slates  or  day  slates,  such  as  are  used  for  roofing  and 
similar  purposes,  the  fissile  property  having  been  imparted  by 
pressure  or  shearing.  Such  forms  pass  by  imperceptible  gra- 
dations into  argillaceous  schists  which  are  classed  with  the  met- 
amorphic  rocks.  (See  p.  170.)  The  argillites  are,  as  a  rule, 
among  the  most  indestructible  of  rocks,  since  they  are  them- 
selves composed  of  the  least  destructible  debris  of  pre-existing 
rocks.  Their  ultimate  chemical  composition  is  much  like  that 
of  the  clays,  and  scarce  any  two  samples  will  show  similar 
results  when  submitted  to  analysis.  The  table  given  below 
shows  the  composition  of  some  schistose  argillites  used  for 
roofing  purposes  from  (I)  Harford  County,  Maryland,  (II) 
Lancaster  County,  Pennsylvania,  and  (III)  Llangynog,  North 
Wales. 


CONSTITUENTS 

I 

II 

III 

Silica  (SiOo) 

58  37  % 

60  32  % 

60  150  % 

Sulphuric  acid  (HoS04) 

0  22 

Alumina  (A^Os) 

21.985 

23.10 

24.20 

Iron  oxides  (FeO)  and  (FesOg)     .     . 
Lime  (CaO)     
Magnesia  (MgO)  
Soda  (Na20)    . 

10.661 
0.30 
1.203 
1  933 

7.05 

0.87 
0  49 

7.65 
I        4  278 

Potash  (K2O)  
Water  (H20)    

4.03 

3.83 

4.08 

3.72 

98.699  % 

99.74% 

99.998% 

Shale  is  a  somewhat  loosely  defined  term,  indicating  struc- 
tural rather  than  chemical  or  mineralogical  composition.  The 
word  is  perhaps  best  used  in  its  adjective  sense,  as  a  shall/ 
sandstone,  or  shaly  limestone.  By  many  authors  it  is  used 
with  reference  more  particularly  to  thinly  stratified  or  lami- 
nated, clayey  rocks.  Many  shales  are  but  the  finer,  more  fissile 
portions  of  sandstone  beds;  such  may  represent  the  off-shore 
or  deep-water  portions  of  arenaceous  sediments,  which,  begin- 
ning with  gravels  near  the  shore-line,  become  gradually  finer 
as  the  distance  from  the  shore  increases,  passing  through  coarse 
to  finer  sands  and  finally  to  sandy  clays  and  silts  as  the  water, 


138 


AQUEOUS   ROCKS 


through  the  lessening  of  its  carrying  power,  lays  down  its  load. 
Or  they  may  represent  later  stages  in  the  cycle  of  sedimenta- 
tion ;  the  finer  silts  brought  down  after  erosion  have  so  far 
reduced  the  level  of  the  land  as  to  greatly  diminish  the  currents 
and  consequent  carrying  power  of  the  seaward-flowing  streams. 
Such  beds,  on  consolidation,  yield  then  what  are  commonly 
known,  in  the  order  of  their  formation,  as  conglomerates,  sand- 
stones, shales  and  argillites,  or  clay  slates,  the  shales  occu- 
pying, both  in  texture  and  composition,  a  position  intermediate 
between  the  argillites  and  sandstones. 

The  following  table  will  serve  to  show  the  varying  character 
of  the  rocks  included  under  this  name.  Those  such  as  given 
in  columns  I  and  II  carry  their  sulphur  in  combination  with 
iron,  as  iron  pyrites  (FeS2).  This,  on  decomposing,  through 
the  action  of  meteoric  waters,  yields  iron  sesquioxides  and  sul- 
phuric acid,  the  latter  combining  with  a  portion  of  the  alumina 
in  the  rock  to  form  sulphate  of  aluminum,  or  common  alum. 
Hence  they  have  been  called  alum  shales. 


CONSTITUENTS 

I 

II 

Ill 

Silica  (Si02)     

50.13% 

72.40% 

66.96  % 

Alumina  (Al20s) 

10.73 

16.45 

15.626 

Iron  sesquioxide  (Fe20s) 

5.78 

1.05 

8.38 

Lime  (CaO)                        

0.40 

0.17 

0.493 

Magnesia  (MgO)  

1.00 

1.48 

0.677 

Potash  (K20)  
Soda  (Na20) 

5.08 
0.53 

3.295 
0.628 

Sulphur  (S) 

4.02 

1.21 

Carbon  (C)       

22.83 

Undet. 

3.787 

Water  (H2O)  1      

2.21 

Undet. 

Phosphoric  acid  (P2O5)     

0.154 

I.  An  alum  shale  from  Garnsdorf,  near  Saalst'eld.  II.  An  alum  shale  from 
Bornholm.  III.  A  "marly  shale  "  from  Breckenridge  County,  Kentucky. 

The  name  till  or  boulder  clay  is  given  to  a  sandy  clay  of 
glacial  origin  and  consisting  of  the  usual  indefinite  mixture. 
Professor  W.  O.  Crosby,  who  has  studied  the  composition  of 
the  normal  till  of  the  Boston  Basin,  reports  it  as  composed, 
exclusive  of  the  larger  pebbles,  of  "about  25%,  or  one-fourth, 
of  coarse  material  which  may  be  classed  as  gravel ;  about  20  %, 

1  Ignition. 


CALCAREOUS   FRAGMENTAL   ROCKS  139 

or  one-fifth,  of  sand;  40  to  45  %  of  extremely  fine  sand,  or  rock 
flour,  and  less  than  12  %  of  clay."  l 

Laterite  is  a  red,  ferruginous  residual  clay  found  in  tropic 
and  semitropic  regions.  (See  p.  310.)  Catlinite,  or  Indian 
pipe-stone,  is  an.  indurated  clay  rock  formerly  used  by  the  Da- 
kota Indians  for  pipe  material.  The  name  porcellainite  has 
been  given  to  a  compact  porcelain-like  rock  consisting  of  clay 
indurated  by  igneous  agencies.  The  name  wacke  is  sometimes 
used  to  designate  an  earthy  or  compact,  dark-colored  clayey 
material  resulting  from  the  decomposition  in  situ  of  basaltic 
rocks.  Adobe  is  the  name  given  to  a  calcareous  clay  of  a 
general  gray-brown  or  yellowish  color,  very  fine  grained  and 
porous,  and  which  is  widely  distributed  throughout  the  more 
arid  regions  of  the  West.  It  is  described  in  greater  detail 
under  the  head  of  soils  (p.  333).  Loess  is  a  somewhat  similar 
material  forming  the  surface  soil  over  wide  areas  in  the  Missis- 
sippi valley,  and  at  times  sufficiently  plastic  for  brick  making. 
(See  also  p.  327.) 

(3)  The  Calcareous  Group.  —  Here  are   brought  together  a 
small  series  of  fragmental  rocks  composed  mainly  of  calcareous 
material,  but  of  which  the  organic  nature,  if  such  it  had,  is  not 
apparent.     These  rocks  form  at  times  beautifully  brecciated 
marbles.    Their  structure  may  be  best  comprehended  by  remem- 
bering that  the  original  beds,  whether  crystalline  or  amorphous, 
whether  f ossilif erous   or  originating   as   chemical  precipitates, 
have  by  geological  agencies  been  crushed  and  shattered  into  a 
million  fragments,  and  then,  by  infiltration  of  lime  and  iron- 
bearing  solutions,  been  slowly  cemented  once  more  into  solid 
rock.     The  composition  is  essentially  the  same  as  the  ordinary 
sedimentary  limestones  and  need  not  be  further  dwelt  upon 
here.     It  may  be  stated,  however,  that  owing  to  the  softness 
and  ready  solubility  of  their  materials  limestones  do  not,  on 
breaking  down,  except  under  rare  instances,  give  rise  to  exten- 
sive beds  of  arenaceous  rocks,   as  do  the  siliceous  varieties. 
One  of  the  best  known  rocks  of  this  group  is  the  breccia  marble 
near  Point  of  Rocks  in  Maryland,  which  has  been  used  in  the 
United  States  Capitol  building  at  Washington. 

(4)  The  Volcanic  Group  :    Tuffs.  —  Under  this  head  are  in- 
cluded a  great  variety  of  fragmental  rocks,  composed  of  the 
more  or  less  finely  comminuted   materials   ejected   from  vol- 

i  Proc.  Boston  Society  of  Natural  History,  Vol.  XXV,  1800,  p.  123. 


140  AQUEOUS   ROCKS 

canoes  as  ashes,  dust,  sand,  and  lapilli.  Some  of  them  are  made 
up  of  minute  shreds  of  pumiceous  glass.  These  occur,  in  many 
instances,  interbedded  with  lava  flows  of  the  same  lithological 
nature,  and  which  are  a  product  of  the  same  periods  of  vol- 
canic activity,  the  eruption  of  molten  lava  being  accompanied 
by  intervals  of  explosive  action,  during  which  only  fragmental 
material  was  ejected.  To  such  materials  the  name  ^/roclastic 
(Greek  Trf/oo?,  fire)  is  appropriately  given. 

The  lithological  character  of  the  materials  varies  almost 
indefinitely,  and  only  very  general  names  are  given  them  in 
the  majority  of  cases.  The  name  tuff  or  tuff  a  is  given  to  the 
entire  group  of  volcanic  materials  formed  as  above,  and  also 
by  some  authorities  to  fragmental  rocks  resulting  from  the 
breaking  down  and  reconsolidation  of  older  volcanic  lavas. 
It  would  seem  advisable  to  designate  these  last,  as  has  F. 
Lowinson-Lessing,1  as  pseudotuffs  or  tuffoids. 

The  names  volcanic  ashes,  sand,  and  dust  are  applied  to 
the  finer  of  these  volcanic  materials,  and  lapilli  or  rapilli  to  the 
coarser  fragments. 

The  dusts  and  sands  are  not  infrequently  composed  of 
minute  shreds  of  volcanic  glass,  which  were  blown  from  the 
volcanic  vents  and  carried  unknown  distances,  to  be  ultimately 
deposited  as  stratified  beds  in  comparatively  shallow  water. 
Such  are  described  more  in  detail  under  the  head  of  ^Eolian 
rocks  (p.  153).  The  term  trass  is  used  to  designate  a  compact 
or  earthy  fragmental  rock  composed  of  pumice  dust,  in  which 
are  embedded  fragments  of  trachytic  and  basaltic  rocks,  car- 
bonized wood,  etc.,  and  which  occupies  some  of  the  valleys  of 
the  Eifel.  Peperino  is  a  tufaceous  rock  composed  of  fragments 
of  basalt,  leucite,  lava,  and  limestone,  with  abundant  crystals 
of  augite,  mica,  leucite,  and  magnetite.  It  occurs  among  the 
Alban  Hills,  near  Rome,  Italy.  Palagonite  tuff  is  composed  of 
dust  and  fragments  of  basaltic  lava,  with  pieces  of  a  pale  yellow, 
green,  reddish,  or  brownish  glass  called  palagonite.  The  general 
name  of  volcanic  mud  is  given  to  the  finely  comminuted  volcanic 
material  which  in  a  more  or  less  pasty  or  liquid  condition  is  thrown 
from  volcanic  vents  during  the  incipient  stages  of  eruption. 

The  tuffs  are  as  a  rule  more  or  less  distinctly  stratified,  of 
very  uneven  texture,  and  with  rarely  a  pisolitic  structure. 
They  are  found  associated  with  volcanic  rocks  of  all  ages,  and 
1  Tschermaks  Min.  u.  Petrog.  Mittheilungen,  Vol.  IX,  1889,  p,  530. 


VOLCANIC   TUFFS 


141 


at  times  so  highly  metamorphosed  as  to  render  the  original 
nature  of  some  doubt.  Certain  English  authorities  have  con- 
tended that  a  part  of  the  so-called  argillites  and  lire  clays  were 
of  finely  comminuted  volcanic  materials. 

The  composition  of  the  tuffs  naturally  varies  with  that  of  the 
character  of  the  lava  from  which  they  were  derived.  Being 
in  a  more  or  less  finely  comminuted  condition,  often  porous 
and  readily  permeated  by  water  or  rootlets,  they  undergo  de- 
composition, forming  soils  the  character  of  which  is  dependent 
to  some  extent  upon  their  lithological  nature.  The  following 
table  shows  the  varying  composition  of  rocks  of  this  class :  — 


o" 

"S  ,*-  - 

s 

°« 

o 

0 

KINDS  AND  LOCALITIES 

33 

15 

& 

!§ 

•S 

C3 

r 

,| 

X 

1 

IS 

|S 

® 

s 

te 

1 

^0 

42 

o 

| 

JS  * 

3 

O 

02 

*•< 

&, 

3 

s 

Oj 

^ 

"2.    0 

H 

Pozzuolana,  Naples, 

% 

% 

% 

% 

% 

% 

% 

% 

% 

Italy      .... 

59.144 

21.28 

4.76 

1.90 

4.37 

6.2*3 

100.24 

Tuff,     Crater     of 

Monte      Nuova, 

Chlorine 

Italy     .... 

56.31 

15.23 

7.11 

1.74 

1.36 

6.54 

4.84 

6.12 

0.27 

100.22 

Trass,  Andernach, 

v  ,  ' 

Prussia.     .     .     . 

54.00 

16.50 

6.10 

4.00 

0.70 

10.00 

7.00 



99.00 

Tuff,  Lacher   See, 

Prussia      .     .     . 

60.49 

19.95 

9.37 

3.12 

1.43 

3.40 

1.33 

99.09 

(2)    ROCKS   COMPOSED   MAINLY   OF  DEBRIS   FROM   PLANT   AND 

ANIMAL   LIFE 

(1)  The  Siliceous  Group :  Infusorial  or  Diatomaceous  Earth.— 
This  is  a  fine  white  or  pulverulent  rock,  composed  mainly  of 
the  minute  shells,  or  tests,  of  diatoms,  and  often  so  soft  and 
friable  as  to  crumble  readily  between  the  thumb  and  fingers. 
It  occurs  in  beds  which,  when  compared  with  other  rocks  of 
the  earth's  crust,  are  of  comparatively  insignificant  proportions, 
but  which  are  nevertheless  of  considerable  geological  impor- 
tance. Though  deposits  of  this  material  are  still  forming,  and 
have  been  formed  in  times  past  at  various  periods  of  the  earth's 
history,  they  appear  most  abundantly  associated  with  rocks 
belonging  to  the  Tertiary  formations. 

The  beds  are  wide-spread,  and  some  of  them  of  economic 
importance  as  a  source  of  tripoli,  absorbents  for  nitro-glycerine 


142 


AQUEOUS   ROCKS 


compounds,  non-conducting  materials,  etc.  A  deposit  in  Biln, 
Bohemia,  is  some  14  feet  in  thickness,  and  is  estimated  by 
Ehrenberg  to  contain  40,000,000  shells  to  every  cubic  inch. 
Beds  occur  in  the  United  States  at  South  Beddington,  Maine  ; 


FIG.  10.  —  Section  through  lake  basin  showing  the  formation  of  infusorial  earth, 
a,  bed  rock ;  bb,  floating  peat ;  cc,  decayed  peat ;  d,  infusorial  earth. 

Lake  Umbagog,  New  Hampshire ;  in  Morris  County,  New  Jer- 
sey ;  near  Richmond,  Virginia ;  in  Calvert  and  Charles  coun- 
ties, Maryland;  in  New  Mexico;  Graham  County,  Arizona;  near 
Reno,  Nevada,  and  in  various  parts  of  California  and  Oregon. 

The  New  Jersey  deposit  covers  about  3  acres,  and  varies 
from  1  to  3  feet  in  thickness  ;  the  Richmond  bed  extends  from 
Herring  Bay,  on  the  Chesapeake,  to  Petersburgh,  Virginia,  and 
is  in  some  places  30  feet  in  thickness ;  the  New  Mexico  deposit 
is  some  6  feet  in  thickness  and  has  been  traced  some  1500  feet. 
Professor  Leconte  states  that  near  Monterey,  in  California,  is 
a  bed  some  50  feet  in  thickness,  while  the  geologists  of  the 
Fortieth  Parallel  Survey  report  beds  not  less  than  300  feet  in 
thickness,  of  a  pure  white,  pale  buff,  or  canary-yellow  color, 
as  occurring  near  Hunter's  Station,  west  of  Reno,  Nevada. 

The  earth  is  used  mainly  as  a  polishing  powder,  and  is  some- 
times designated  as  tripolite.  It  has  also  been  used  to  some 
extent  to  mix  with  nitro-glycerine  in  the  manufacture  of  dyna- 
mite. Chemically  the  rock  is  impure  opal,  as  will  be  seen  from 
the  following  analyses  made  on  samples  from  (I)  Lake  Umba- 
gog, New  Hampshire,  (II)  Morris  County,  New  Jersey,  and 
(III)  Paper  Creek,  Maryland:  — 


CONSTITUENTS 

I 

II 

ill 

Silica  (SiO2)     .     . 

80  53  % 

80  60  % 

81.53% 

Iron  oxides  (Fe203  and  FeO)     .     .     . 
Alumina  (Al20s)  

1.03 

5.89 

3.84 

3.33 
3.43 

Lime  (CaO)      .... 

0  35 

0  58 

2  61 

Water  (H20)    

11  05 

14  00 

6.04 

Organic  matter     

0  98 

99.38% 

99.02% 

96.94% 

Number  III  showed  also  small  amounts  of  potash  and  soda. 


PLATE    12 


;*«-v  ^ 


,.^^^-^d^  ?  v^*V^-':  y^^''^'- '  ^;^^:^t 

tl^*^^-^,*;  ^  ;i!-^^^w'^Vut-V 4^.^.'^  .^v^J^ 
^3fc5S-:.-.  "'^Pt^C^^i. !^, w:^^--^-i^v;>^^<^ -^ 


^ji^M^^«i^ 

,^-^i  ^  ;^SM^:&^« 


PIG.  1.  Pisolitic  limestone. 


FIG.  2.  Oolitic  limestone. 


LIMESTONES  143 

(2)  The  Calcareous  Group.  —  These  rocks  are  made  up  of  the 
more  or  less  fragmental  remains  of  molluscs,  corals,  and  other 
marine  and  fresh-water  animals.  Many  of  them  are  but  con- 
solidated beds  of  calcareous  mud,  full  of  more  or  less  fragment- 
ary shells  or  casts  of  shells,  as  shown  in  Fig.  1,  PI.  11.  The 
name  coquina  (Spanish  for  shell)  is  given  to  such  as  that 
shown  in  Fig.  2,  PI.  11,  from  St.  Augustine,  Florida.  The 
rock,  it  will  be  observed,  is  composed  almost  wholly  of  very 
perfect  shells  of  a  bivalve  mollusc,  loosely  cemented  by  calcare- 
ous materials  in  a  finely  divided  condition.  From  such  forms 
as  these  we  have  all  possible  gradations  to  compact  crystalline 
limestone.  Special  names  are  often  given  these  calcareous 
rocks,  designating  the  character  of  materials  from  which  they 
are  derived.  Coral  and  shell  limestones,  as  the  names  denote, 
are  composed  mainly  of  the  debris  from  these  organisms.  In 
like  manner  such  names  as  crinoidal,  fusulina,  etc.,  are  applied. 

Lumachelle  is  the  name  given  to  a  shell  limestone  from  the 
Tyrol,  in  which  the  shells  still  retain  their  pearly  lining  and 
original  beauty.  Nummulitic  limestone  carries  fossil  nummulites. 
Rocks  of  this  type  were  used  in  the  construction  of  the  pyramids 
of  Cheops.  Chalk  is  a  fine-grained,  white,  pulverulent  rock, 
composed  of  finely  broken  shells  of  marine  molluscs,  among 
which  minute  foraminifera  are  abundant.  Shell  sand  is  a 
loose  aggregate  of  shell  fragments,  formed  on  sea-beaches  by 
the  action  of  the  winds  and  waves.  On  certain  Hawaiian 
beaches,  such  sands  give  out  a  distinct  note,  or  peculiar  crunch- 
ing sound  when  walked  over,  or  even  when  shaken  in  a  closed 
vessel,  and  are  popularly  known  as  sounding,  or  singing,  sands. 
The  property  is  manifested  only  when  the  sand  is  dry  and  is 
assumed  to  be  due  to  the  minute  air  cavities  enclosed  by  the 
shells.  Oolitic  and  pisolitic  limestones,  as  previously  noted,  are 
made  up  of  rounded  concretionary  masses  of  calcium  carbonate, 
and  are  in  part  of  mechanical  origin,  and  in  part  chemical  de- 
posits (PL  12). 

The  microscopic  structure  of  an  oolitic  limestone  from  Prince- 
ton, in  Caldwell  County,  Kentucky,  is  shown  in  the  accompany- 
ing figure  (p.  144).  It  will  be  noticed  that  the  first  step  in  the 
formation  of  this  stone  was  the  deposition  of  concentric  coat- 
ings of  lime  about  a  nucleus  which  is  sometimes  nearly  round, 
but  more  frequently  quite  angular  and  irregular.  After  the 
concretions  were  completed  there  were  formed  in  all  cases  about 


144 


AQUEOUS  HOCKS 


each  one,  narrow  zones  of  minute  radiating  crystals  of  clear, 
colorless  calcite;  then  the  larger  crystals  formed  in  the  inter- 
stices. The  nuclei  are  composed  in  some  cases  of  single  frag- 
ments or,  again,  of  a  group  of  fragments.  Certain  of  the  oolites 
present  no  distinct  concentric  structure,  but  appear  as  mere 
rounded  masses  merging  gradually  into  the  crystalline  inter- 
stitial portions.  Recent  microscopic  studies  have  tended  to 
show  that  many  of  the  oolitic  limestones  owe  their  structure  to 
the  lime-secreting  power  of  microscopic  algie.1 

Limestones  vary  almost  indefinitely  in  structure  and  color. 
From  the  soft  tufaceous  or  highly  fossiliferous  varieties  there 

is  a  constant  gradation 
to  dense  compact  rocks 
breaking  with  a  conchoi- 
dal  or  splintery  fracture 
and  the  true  nature  of 
which  is  sometimes  to  be 
ascertained  only  by  chem- 
ical tests.  There  is  a  like 
variation  in  color.  White 
through  all  shades  of  gray 
to  black  is  common,  and 
more  rarely  occur  yellow, 
brown,  pink,  or  red  vari- 
eties, the  colors  depend- 
ing on  organic  matter  and 
metallic  oxides,  mainly 
ferruginous. 

Owing  to  the  readiness  with  which  calcium  carbonate 
undergoes  crystallization,  even  at  ordinary  temperatures,  few 
limestones  are  wholly  amorphous,  but  grade  insensibly  into 
holocrystalline  varieties  such  as  are  classed  with  the  metamor- 
phic  rocks.  The  name  marble  is  given  to  such  limestones  as 
are  of  sufficiently  close  texture  to  take  a  polish  and  of  such 
colors  as  to  make  them  desirable  for  ornamental  work.  A  large 
proportion  of  the  marbles  belong,  however,  to  the  metamor- 
phic  group.  (See  p.  162.)  Figure  12  shows  the  microscopic 
structure  of  a  dark  gray,  variegated,  highly  fossiliferous  lime- 
stone belonging  to  the  Cincinnati  group,  near  Hamilton,  Ohio. 
It  is  a  natural  result  of  their  method  of  formation  that  few 
1  American  Geologist,  Vol.  X,  No.  5,  1892. 


FIG.  11.  —  Microstructure  of  oolitic  limestone. 


LIMESTONES 


145 


limestones  are  of  pure  calcium  carbonate.  A  portion  of  the 
calcium  is  not  infrequently  replaced  by  magnesium,  giving  rise 
to  magnesian  limestones,  or  when  the  proportion  of  magnesia 
rises  to  45. G5  %  to  dolomite.  This  last  can  as  a  rule  be  distin- 
guished from  limestone  only  by  its  increased  hardness  (3.5-4.5) 
and  specific  gravity  (2.8-2.95).  Frequently  chemical  tests  are 
•necessary,  limestone  effervescing  readily  when  treated  with 
dilute  hydrochloric  acid,  while  dolomite  is  unacted  upon. 

Mechanically  included  materials,  as  sand  and  clay,  are  com- 
mon, giving  rise  to  siliceous  and  argillaceous  varieties.  The  so- 
called  hydraulic  limestone 
is  one  containing  10  % 
and  upwards  of  these 
impurities,  and  which, 
when  burnt  and  ground, 
forms  a  cement  charac- 
terized by  its  property 
of  setting  under  water. 
Many  limestones,  like  the 
dolomitic  varieties  in 
Cook  County,  Illinois, 
contain  so  large  a  pro- 
portion of  bituminous 
matter  as  to  give  off  a 
distinct  odor  of  petro- 
leum when  struck  with  a 
hammer,  or  even  to  be- 
come blackened  on  the 
surface  by  its  exudation  when  exposed  to  the  weather.  Others 
contain  phosphatic  matter,  and  pass  by  insensible  gradations 
through  what  are  known  as  phosphatic  limestones  to  true  phos- 
phates (phosphorites,  etc.). 

In  chemical  composition  the  limestones  vary,  like  other  sedi- 
mentary rocks,  almost  indefinitely,  as  will  naturally  be  inferred 
from  what  is  said  above.  As  a  general  rule,  those  varieties, 
which  have  been  formed  in  deep  waters  and  at  a  distance  from 
the  shores,  will  be  of  greatest  purity,  since  less  likely  to  have 
become  contaminated  through  detrital  materials  washed  in  from 
the  land.  Even  these  may,  however,  be  intermingled  to  a  very 
considerable  extent  with  the  fine  siliceous  and  ferruginous  mat- 
ter, such  as  deep-sea  dredgings  have  shown  to  be  common  to 


FIG.  12.  —  Microstructure  of  fossiliferous 
limestone. 


146 


AQUEOUS   ROCKS 


our  modern  sea-bottoms,  and  which  are  assumed  to  be  in  part  at 
least  of  volcanic  origin.  (See  under  JEolian  Rocks,  p.  153.)  The 
following  table  will  give  some  idea  of  the  wide  range  in  chemi- 
cal composition  to  be  found  in  rocks  of  this  class  :  — 


2 

2 

O 

c 

s    • 

"S  5  S 

s 

;-    2    S 

5  ~r  a 

c  s 

l£> 

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s 

CONSTITUENTS 

n 

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a  „ 

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*i  _o  "S  ^ 

|§="ci 

Igl 

>.,§  fl 

£S^S 

fSS^fc 

S^5-i 

Pt^^ 

Carbonate  of  lime  (CaC03)  .     .     . 

98.00  % 

54.62% 

41.48% 

72.95-% 

96.60  % 

Carbonate  of  magnesium  (MgCOs) 

.... 

45.04 

24.55 

3.84 

0.13 

Oxides  of  iron  (FeO  and  Fe2O3)    . 
Oxide  of  aluminum  (A1203)      .     . 

J0.23 

J4.03 

1.34 
4.50 

0.98 

Silica  (Si02)  and  insol.  silicates    . 

0.57 

.... 

29.93 

14.79 

0.50 

Potash  (K20) 

1  22 

0  31 

Soda  (Na20)                      .... 

1  12 

0.40 

Water  (H20) 

0.96 

Sulphate  of  lime  (CaS03)      .     .     . 

1.75 

Organic  matter    

1.46 

Totals 

98  57  °/ 

99  89  °/ 

98  33  % 

100  64  % 

99  88  % 

Researches  by  the  Kentucky  Geological  Survey  have  shown 
that  the  older  limestones  are,  as  a  general  rule,  richer  in  soda, 
phosphoric  acid,  and,  when  non-magnesian,  in  lime  carbonate, 
than  are  the  younger  more  recently  formed,  and  correspondingly 
poorer  in  silica  and  insoluble  silicates.  This  inverse  ratio  is 
shown  in  the  table  on  the  opposite  page,  in  which  the  rocks  are 
arranged  by  geological  horizons,  the  oldest  at  the  bottom. 

The  name  shell  marl,  or  merely  marl,  is  given  to  an  illy  denned, 
often  arenaceous,  soft  and  earthy  rock  consisting  essentially  of 
shell  material  in  a  more  or  less  fragmental  condition,  and  usu- 
ally intermixed  with  more  or  less  clayey  matter  or  siliceous 
sand  and  silt.  Geikie 1  would  limit  the  term  to  fresh-water 
accumulations  of  remains  of  mollusca,  entomostra ,  and  fresh- 
water algse,  but  unfortunately  the  word  has  not  been  so  used 
in  much  of  the  literature  extant.  These  marls,  being  easily 
decomposed,  and  on  account  of  their  occasional  richness  in 
phosphoric  acid,  or,  perhaps,  merely  on  account  of  the  lime 
they  contain,  are  of  value  as  fertilizers.  The  following  analy- 
ses of  North  Carolina  marls,  consisting  largely  of  comminuted 

1  Text-book  of  Geology.     3d  ed. 


COMPOSITION  OF   LIMESTONES 


147 


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LIMESTONES 

nposition  of  G  Coal-measure  limestone 
nposition  of  10  upper  sub-Carbonifer( 

3  

nposition  of  3  lower  sub-Carbonifen 
5  (hydraulic)  
nposition  of  0  Ulack  Slate  limestones  . 

nposuum  01  -i  uaruoniieruus  iimesLon 
h  2  are  magnesian  (hydraulic  ?)  .  . 
re  non-magnesian  
nposition  of  14  Niagara  group  (or  Up 
limestones  
nposition  of  3  Clinton  group  limeston* 
nposition  of  3  upper  Hudson  group  lii 

mposition  of  7  middle  Hudson  grc 
us  mudstone")  
nposition  of  9  lower  Hudson  limeston< 

nposition  of  7  Trenton  limestones  (n 
n)  
nposition  of  11  Trenton  limestones  (m 

nposition  of  2  Bird's-eye  limestones 
nposition  of  3  Chazy  limestones 

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148 


AQUEOUS   ROCKS 


shells  and  sometimes  coprolite  nodules,  will  serve  to  show  the 
widely  varying  character  of  the  materials  grouped  under  this 


name. 


CONSTITUENTS 

I 

II 

III 

IV 

V 

VI 

VII 

Silica                                  .     . 

% 
6.97 

% 
61  61 

% 
18  84 

% 

58  25 

% 
25  28 

39  36 

% 
5  65 

Oxide  of  iron  and  alumina   . 
Lime  

0.86 
47.62 

2.80 
19.60 

2.72 
41.48 

11.28 
13.49 

3.02 
37  52 

3.47 

28  96 

3.30 

48  51 

Magnesia    
Potash 

1.03 
037 

0  56 

0.12 
0  22 

0.16 
0  75 

1.96 
0  23 

Soda 

0.15 

0.09 

0  25 

0  17 

0  30 

Phosphoric  acid  
Sulphuric  acid     

0.19 
0.41 

0.06 

0.18 
0.64 

.... 

0.40 
0  40 

0.11 

0  18 

trace 
0  31 

Carbonic  acid  
Organic  matter  and  water     . 

38.15 
4.25 

15.37 

32.07 
3.42 

10.59 

29.02 
2.98 

22.73 
4.11 

39.80 
0.60 

(3)  The  Carbonaceous  Group:  Peat,  Lignite,  and  Coal.  —  Here 
are  included  a  variety  of  more  or  less  oxygenated  hydrocarbons 
varying  widely  in  physical  and  chemical  properties,  but  alike 
in  originating  from  decomposing  plant  growth  protected  from 
the  oxidizing  influences  of  the  air.  Plants,  when  decomposing 
upon  the  surface  of  the  ground,  give  off  their  carbon  to  the 
atmosphere  in  the  shape  of  carbonic  acid  gas  (CO2),  leaving 
only  the  strictly  inorganic  or  mineral  matter  behind.  When, 
however,  protected  from  this  oxidizing  influence  by  water,  or 
other  plant  growth,  decomposition  is  greatly  retarded,  varying 
portions  of  the  carbonaceous  and  volatile  matters  are  retained, 
and  the  material  becomes  slowly  converted  into  coal.  Accord- 
ing to  the  amount  of  change  that  has  taken  place  in  the  original 
plant  material,  the  amount  of  volatile  matter  still  retained  by  it, 
its  hardness  and  burning  qualities,  several  varieties  are  recog- 
nized, which,  however,  pass  into  each  other  by  insensible  gra- 
dations. 

Peat  is  the  plant  matter  in  its  least  changed  condition. 
It  results  from  the  gradual  accumulation  in  bogs  and  marshes 
of  growths  consisting  mainly  of  sphagnous  mosses,  a  low  order 
of  plants  having  the  faculty  of  continuing  in  growth  upwards 
as  they  die  off  below.  In  this  way  the  deposits  often  assume 
a  very  considerable  thickness.  Where  sufficiently  thick,  the 
1  Geology  of  North  Carolina,  Vol.  I,  1875,  p.  195. 


PEAT  AND   LIGNITE 


149 


lower  portions  have  sometimes  been  converted  into  a  dense 
brownish  black  mass  somewhat  resembling  true  coal.  The 
deposits  of  peat  are  all  comparatively  recent  and  occur  only 
in  humid  climates.  They  are  developed  to  an  enormous 
extent  in  Ireland  —  about  one-seventh  of  the  entire  country 
being  covered  by  them  —  and  average  in  some  cases  25  feet 
in  thickness.  They  are  also  abundant  in  Europe  and  various 
parts  of  North  America.  In  Europe,  and  especially  in  Ireland, 
the  material  is  extensively  utilized  for  fuel,  and  there  would 
seem  no  good  reason  for  not  so  utilizing  it  in  America.  An 
impure  variety  containing  a  considerable  quantity  of  siliceous 
sand,  and  locally  known  as  "  muck,"  is  used  as  a  fertilizer  and 
for  "  multching  "  throughout  New  England.  Below  are  given 
the  results  of  analyses  of  (I)  peat  from  the  bog  of  Allan, 
Ireland,  (II)  Maine  (United  States),  and  (III)  Commander 
Islands  in  Bering  Sea. 


CONSTITUENTS 

I 

11 

III 

Carbon     

61.04% 

21.00% 

60.48% 

Volatile  matter     

37.53 

72.00 

39.53 

Ash     

1.83 

7.00 

3.30 

Lignite,  or  brown  coal,  is  the  name  given  to  a  brownish  black 
variety  characterized  by  a  brilliant  lustre,  conchoidal  fracture, 
and  brown  streak.  Such  contain  from  55  %  to  65  %  of  carbon, 
and  burn  easily,  with  a  smoky  flame,  but  are  inferior  to  the 
true  coals  for  heating  purposes.  They  are  also  objectionable 
on  account  of  the  soot  they  create,  and  their  rapid  disintegra- 
tion and  general  deterioration  when  exposed  to  the  air.  They 
occur  in  beds  under  conditions  similar  to  the  true  coals,  but 
are  of  more  recent  origin.  The  lignitic  coals  of  the  regions  of 
the  United  States,  west  of  the  Mississippi  River,  are  mainly  of 
Laramie  (Upper  Cretaceous)  age,  and,  as  a  rule,  show  easily 
recognizable  traces  of  their  organic  origin,  such  as  compressed 
and  flattened  stem  and  trunks  of  trees  with  traces  of  woody 
fibre. 

Bituminous  Coal.  —  Under  this  name  are  included  a  series  of 
compact  and  brittle  products  in  which  no  traces  of  organic 
remains  are  to  be  seen  on  casual  inspection,  but  which,  under 


150 


AQUEOUS   KOCKS 


the  microscope,  often  show  traces  of  woody  fibre,  spores  of 
lycopods,  etc.  These  coals  are  usually  of  a  brown  to  black 
color,  with  a  brown  or  gray  brown  streak,  breaking  with  a 
cubical  or  conchoidal  fracture,  and  burning  readily  with  a 
yellow,  smoky  flame.  They  contain  from  35  %  to  70  %  of  fixed 
carbon,  18  %  to  60  %  of  volatile  matter,  and  from  2  %  to  20  % 
of  water,  and  only  too  frequently  show  traces  of  sulphur  due 
to  included  iron  pyrites.  Several  varieties  of  bituminous  coals 
are  recognized,  the  distinctions  being  based  upon  their  manner 
of  burning.  Coking  coals  are  so  called  from  the  facility  with 
which  they  may  be  made  to  yield  coke ;  such  give  a  yellow 
flame  in  burning,  and  make  a  hot  fire.  They  are  soft,  and 
break  with  a  cubical  fracture.  Other  varieties  of  apparently 
the  same  composition  and  general  physical  properties,  cannot, 
for  some  unexplained  reason,  be  made  to  yield  coke,  and  are 
known  as  non-coking  coals.  Cannel  coal  has  a  very  compact 
structure,  breaks  with  a  conchoidal  fracture,  has  a  dull  lustre, 
ignites  easily,  and  burns  with  a  yellow  flame.  It  does  not  coke. 
Its  chief  characteristic  is  the  large  amount  of  volatile  matter 
given  off  when  heated,  whereby  it  is  rendered  of  particular 
value  for  making  gas.  Before  the  discovery  of  petroleum  it 
was  used  for  the  distillation  of  oils.  Below  is  given  the  com- 
position of  (I)  a  coking  coal  from  the  Connelsville  Basin  of 
Pennsylvania,  and  (II)  a  cannel  coal  from  Kanawha  County, 
West  Virginia.1 


CONSTITUENTS 

I 

II 

Water  

1.05% 

Volatile  matter     

29.885 

Fixed  carbon    

57.754 

58.00% 

Ash      

9  895 

23  50 

Sulphur   

1  339 

18.50 

100.00  % 

100.00  % 

Anthracite  Coal.  —  This  is  a  deep  black,  lustrous,  hard  and 
brittle  variety,  and  represents  the  most  highly  metamorphosed 
variety  of  the  coal  series.  Such  have  been  generally  regarded 
as  bituminous  coals  from  which  a  very  large  proportion  of  the 

1  F.  P.  Dewey,  Bull.  42,  U.  S.  National  Museum,  1891. 


THE   PHOSPHATES  151 

volatile  constituents  have  been  driven  off  by  the  agencies  in- 
volved in  the  production  of  mountain  systems,  or  by  the  heat 
incident  to  the  injection  of  igneous  rocks.  Traces  of  organic 
nature  are  almost  entirely  lacking  in  the  matter  of  the  anthra- 
cite itself,  though  impressions  of  ferns,  lycopods,  sigillaria,  and 
other  coal-forming  plants  are  frequently  associated  with  the 
beds  in  such  a  manner  as  to  leave  little  doubt  as  to  their  origin. 
Anthracite  is  ignited  with  difficulty  and  burns  with  little  flame, 
but  makes  a  hot  fire.  Below  is  given  the  average  composition 
of  anthracite  from  the  Kohinoor  Colliery,  Shenandoah,  Penn- 
sylvania.1 

Water 3.163% 

Volatile  matter 3.717 

Fixed  carbon 81.143 

Sulphur 0.899 

Ash 11.078 

100.000  % 

Like  the  other  coals,  anthracite  occurs  in  true  beds,  but  is 
confined  mostly  to  rocks  of  the  Carboniferous  age.  Thin  seams 
of  anthracite  sometimes  occur  in  Devonian  and  Silurian  rocks, 
but  which  are  too  small  to  be  of  economic  value.  Rarely 
coals  of  more  recent  geological  horizon  have  been  found  locally 
altered  into  anthracite  by  the  heat  of  igneous  rocks.  Through 
a  still  further  metamorphism,  whereby  it  loses  all  its  volatile 
constituents,  coal  passes  over  into  graphite. 

The  principal  anthracite  coal  regions  of  the  United  States  are 
in  eastern  Pennsylvania.  From  here  westward  throughout  the 
interior  states  to  the  front  range  of  the  Rocky  Mountains  the 
coals  are  all  soft,  or  bituminous  coals.  Those  of  the  Rocky 
Mountain  regions  proper  are  largely  lignitic,  passing  into  the 
bituminous  varieties. 

(4)  Phosphatic  Group:  Phosphatic  Sandstone;  Bone  Breccia; 
Guano ;  Coprolite  Nodules.  —  This  is  a  group  of  rocks  limited  in 
extent,  but  nevertheless  of  considerable  economic  importance, 
owing  to  the  high  values  of  certain  varieties  for  fertilizing 
purposes. 

G-uano  consists  mainly  of  the  excrements  of  sea  fowls,  and  is 

to  be  found  in  beds  of  any  importance  only  in  rainless  regions 

like  those  of  the  western  coast  of  South  America  and  southern 

Africa.     The  most  noted  deposits  are  on  small  islands  off  the 

1  Bull.  42,  U.  S.  National  Museum,  1891. 


152  AQUEOUS   ROCKS 

coast  of  Peru.  Immense  flocks  of  sea  fowls  have,  in  the  course 
of  centuries,  covered  the.  ground  with  an  accumulation  of  their 
droppings  to  a  depth  of  sometimes  30  to  80  feet,  or  even  more. 

An  analysis  of  American  guano  gave  :  Combustible  organic 
matter  and  acids,  11.3  %  ;  ammonia  (carbonate,  etc.),  31.7  %  ; 
fixed  alkaline  salts,  sulphates,  phosphates,  chlorides,  etc., 
8.1%;  phosphates  of  lime  and  magnesia,  22.5%;  oxalate  of 
lime,  2.6  %  ;  sand  and  earthy  matter,  1.6  %  ;  water,  22.2  % 
(Geikie). 

Coprolite  nodules  are  likewise  the  excrements  of  vertebrate 
animals ;  those  among  the  Carboniferous  shales  of  the  basin  of 
the  Firth  of  Forth  are  regarded  as  accumulated  excretions  of 
ganoid  fishes. 

Phosphatic  sandstones,  as  the  name  denotes,  are  arenaceous 
rocks  containing  more  or  less  phosphatic  matter.  Inasmuch 
as  the  phosphatic  material  is  derived  largely  by  leaching  and 
segregation,  these  rocks  have  been  already  described  under  the 
head  of  chemical  deposits  (p.  119).  In  the  river  beds  of  the 
Carolinas  are  found  rounded  and  nodular  masses  of  this  nature, 
consisting  of  siliceous  and  calcareous  sand,  with  embedded 
bones,  teeth  of  sharks,  and  other  animal  remains.  Bone  breccia, 
consists  of  fragmental  bones  of  mammals  cemented  by  argil- 
laceous, earthy,  or  calcareous  matter. 


III.    AEOLIAN   ROCKS 

This  group  comprises  a  small  and  comparatively  insignificant 
class  of  rocks  formed  from  materials  drifted  by  the  winds,  and 
more  or  less  compacted  into  rock  masses.  They  are,  as  a  rule, 
of  a  loose  and  friable  texture  and  of  a  fragmental  nature. 
Many  of  the  volcanic  fragmental  rocks  (tuffs)  are  grouped 
here,  their  materials  having  been  thrown  from  the  volcanic 
vents  in  small  fragments  and  drifted  long  distances  by  wind 
prior  to  falling  upon  the  surface  of  the  ground  or  into  the 
water  for  their  final  consolidation. 

One  of  the  most  common  results  of  wind  action  on  the  land 
is  the  production  of  sand-dunes  —  billowy  masses  of  loose  sand 
which,  like  drifts  of  snow,  though  more  slowly,  gradually 
change  their  outlines  and  creep  onward  under  the  restless  goad- 
ing of  the  Avind. 

Such,  owing  to  their  superficial  nature,  recent  origin,  and 
loose  state  of  consolidation,  are  considered  more  in  detail  in 
the  chapter  on  The  Regolith,  p.  299.  On  undergoing  consoli- 
dation, these  dune  sands  may  give  rise  to  sandstones  in  many 
instances  indistinguishable  from  those  of  aqueous  origin,  though 
less  regularly  bedded.  The  finely  disintegrated  shell  and  coral 
material  thrown  up  by  the  waves  on  the  beaches  of  Bermuda 
is  caught  up  by  the  winds  and  drifted  inland,  forming  hills 
which,  in  some  instances,  are  250  feet  in  height.  Being  soluble 
in  the  water  from  rainfalls,  these  become  shortly  reconsolidated 
through  the  deposition  of  lime  carbonate  in  the  interstices  of 
the  fragments,  and  form  thus  the  drift  rock  which  comprises  a 
large  portion  of  the  mass  of  the  islands  above  tide  level. 

The  finely  comminuted  materials  ejected  from  volcanic  vents 
may  be  likewise  transported  by  atmospheric  currents  and,  far 
from  their  source,  again  deposited  in  beds  of  no  insignificant 
proportions.  These,  on  induration,  give  rise  to  fine-grained 
tuffs,  and,  where  the  final  deposition  has  taken  place  in  water, 
to  distinctly  laminated,  fine  white  rocks  the  lithological  nature 

153 


154 


KOCKS 


of  which  can  be  made  out  only  by  means  of  the  microscope. 
Such  are  many  of  the  Pliocene  sandstones  of  Idaho  and  Mon- 
tana.1 The  following  analyses  of  samples  of  tuffs  from  (I) 
Marsh  Creek  Valley,  Idaho ;  and  (II)  Little  Sage  Creek,  Mon- 
tana, will  serve  to  show  their  composition. 


CONSTITUENTS 

I 

II 

Ignition  (H20) 

7.60  % 

7  62  % 

Oxide  of  iron  and  aluminium  (Fe203  and  A12O3)     . 
Silica  (Si02)      

16.22 
68.92 

18.24 
65.56 

Lime  (CaO)      

1.62 

2.58 

Magnesia  (MgO)  

Trace 

0.72 

Soda  (Na2O)     

1.56 

2.08 

Potash  (K20)    

4.00 

3.94 

99.92  % 

100.74% 

1  On  the  Composition  of  Certain  Pliocene  Sandstones  from  Montana  and 
Idaho,  Am.  Jour,  of  Science,  Vol.  XXVII,  1886,  p.  199. 


IV.    METAMORPHIC   ROCKS 

Before  proceeding  to  describe  in  detail  the  metamorphic 
rocks,  it  will  be  well  to  devote  a  brief  space  to  a  discussion 
of  the  processes  by  which  this  metamorphism  has  been  brought 
about. 

The  word  metamorphism  as  used  in  geology  includes  changes 
in  the  structure  of  rocks  induced  through  agencies  in  part 
physical,  and  in  part  chemical,  in  their  nature.  It  is,  in  fact, 
a  very  general  term,  and  indicates  any  transformation  taking 
place  in  the  composition  and  structural  features  of  rocks 
of  any  kind,  whether  sedimentary  or  igneous,  and  from  any 
cause  whatever.  Rocks  laid  down  in  the  form  of  sediments 
may  become  so  deeply  buried  as  to  be  subject  to  intense  heat 
from  the  earth's  interior,  as  well  as  to  pressure  from  weight  of 
the  overlying  material.  In  this  way,  a  partial  or  complete 
fusion  of  the  constituents  takes  place,  which  is  followed  by  a 
crystallization  whereby  the  original  fragmental  nature  may  be 
wholly  or  in  part  obscured.  This  form  of  change  is  included 
under  the  general  name  of  regional  metamorphism.  In  this 
manner,  it  was  once  assumed,  were  formed  the  gneisses,  a  part 
of  the  granites,  and  the  vast  series  of  crystalline  schists  and 
calcareous  rocks  (marbles,  etc.).  It  has,  however,  been  shown 
that  the  banded  and  foliated  structure  shown  by  gneisses  and 
schists  is  not  in  all  cases  necessarily  an  indication  of  an  original 
bedded  structure,  but  may  be  due  to  pressure  acting  through- 
out long  periods  of  time,  and  accompanied  by  the  heat  thereby 
generated.  A  common  and  readily  understood  illustration  of 
this  principle  of  metamorphism  by  pressure  is  offered  by  the 
roofing  slates.  These,  first  laid  down  as  fine  silts,  rarely  show 
their  eminent  cleavages  whereby  they  are  rendered  so  useful  to 
man,  parallel  to  their  original  bedding,  but  inclined  at  any  and 
all  angles  thereto.  In  such  cases  the  bedding  is  not  infre- 
quently indicated  by  the  dark  bands  or  "ribbons"  which  are 
so  evident  on  a  split  surface. 

155 


156  METAMORPHIC   ROCKS 

But  it  is  not  alone  the  fragmental  rocks  which  thus  become 
schistose  under  pressure.  Originally  massive,  igneous  rocks, 
in  regions  of  profound  disturbance  have  been  found  converted 
into  schistose  aggregates,  indistinguishable  from  rocks  ordina- 
rily assumed  to  be  sedimentary.  Thus  the  greenstone  schists 
of  the  Menominee  and  Marquette  regions  of  Michigan  have 
been  shown  by  Williams l  to  be  highly  altered  eruptive  rocks, 
mainly  gabbros,  diabases,  and  diorites,  originally  massive,  but 
now  foliated,  schistose,  and  variously  crumpled  through  the 
squeezing  and  shearing  to  which  they  have  been  subjected  since 
the  period  of  their  first  extrusion.  The  changes  in  these  and 
similar  cases,  is  rarely  purely  physical,  though  at  times  the 
chemical  alterations  may  be  quite  inconspicuous.  The  ulti- 
mate composition  of  the  rock  may  remain  essentially  the  same, 
while  the  method  of  combination  of  its  various  elements  may 
have  undergone  extensive  alteration.  Quartzes  and  feldspars 
may  be  crushed  and  distorted,  drawn  out  into  lens-shaped  and 
variously  elongated  forms,  while  secondary  minerals  like  feld- 
spars, quartz,  zoisite,  garnet,  hornblende,  epidote,  and  the  micas 
may  be  abundantly  generated. 

One  of  the  commonest  results  of  pressure  effects  upon 
igneous  rocks  is  the  conversion  of  augite  or  other  minerals  of 
the  pyroxene  group  into  hornblendes.  The  coarse  hypersthene 
gabbro  occurring  about  Baltimore  is  fpund  locally  altered  into 
a  rock  consisting  essentially  of  a  schistose  aggregate  of  horn- 
blende and  plagioclase  feldspars,  or  what,  on  mineralogical 
grounds,  might  be  classed  as  a  diorite.2  The  chemical  compo- 
sition in  this  case  has  undergone  no  appreciable  change ;  there 
has  been  simply  a  molecular  rearrangement  of  the  particles. 
In  such  cases  proof  of  the  character  of  the  change  that  has 
taken  place  is  usually  found  in  the  fractured  and  otherwise 
distorted  condition  of  many  of  the  constituent  minerals,  as  well 
as  intermediate  stages  of  alteration,  whereby  a  residual  augite 
crystal  is  found  enclosed  in  an  envelope  of  secondary  horn- 
blende, as  shown  in  Fig.  1,  on  p.  40.  To  the  secondary 
minerals  formed  in  this  way  the  technical  name  paramorpJiic 
is  applied.  To  such  changes  as  are  above  described  the  name 
dynamic  metamorphism  is  given. 

The  protrusion  of  a  mass  of  molten  matter  into  the  over- 

1  Bull.  62,  U.  S.  Geol.  Survey,  1890. 

2  Bull.  28,  U.  S.  Geol.  Survey,  1886. 


CONTACT  METAMORPHISM  157 

lying  strata  may  give  rise  to  a  series  of  changes  differing  from 
the  last  in  that  they  are  due  mainly  to  heat  and  to  the  chemical 
action  of  accompanying  vapors  and  solutions.  Since  these 
changes  are  confined  to  limited  areas  along  the  line  of  the 
contacts  between  the  two  bodies,  they  are  defined  as  contact 
metamorphisms.  As  illustrative  of  such  changes,  a  few  cases 
may  be  described. 

Near  Gefrees,  in  Bavaria,  an  eruptive  biotite  granite  has 
been  protruded  into  clay  slates  and  phyllites.  At  the  line 
of  contact  both  phyllites  and  slates  are  converted  into  a 
hard,  compact  blue-black  "  hornfels "  consisting  of  a  crystal- 
line granular  aggregate  of  quartz,  deep  reddish  brown  mica 
(biotite),  a  little  muscovite  and  andalusite.  This  zone,  some 
120  paces  in  width,  is  succeeded  by  a  second  some  380  paces  in 
width  in  which  the  rocks  are  converted  into  andalusite  mica 
schists,  and  this  by  a  third  zone  some  500  paces  wide  in  which 
the  gradually  failing  energy  was  sufficient  only  to  give  rise  to 
a  spotted  mica  schist  (knoten  schiefer),  and  lastly,  a  zone  some 
400  paces  wide  in  which  the  clay  slates  has  become  converted 
into  a  chiastolite  schist,  and  the  phyllites  to  a  biotite-bearing 
variety.  In  all  these  cases  the  chemical  character  of  the  rock 
remains  essentially  the  same.  Through  the  metamorphosing 
action  of  intruded  basic  rocks  crystalline  schists  near  Peekskill, 
New  York,  have  near  the  line  of  contact  become  puckered  and 
filled  with  lens-shaped  eyes  of  quartz  containing  garnets  and 
other  minerals,  while  crystals  of  staurolite,  sillimanite,  cyanite, 
and  garnet  appear,  the  amount  of  change  being  directly  propor- 
tional to  the  nearness  of  the  line  of  contact.  At  the  contact 
the  schistose  structure  is  almost  completely  obliterated  and  the 
schists  become  hard  and  massive,  appearing  more  or  less  fused 
with  the  eruptive,  and  consist  of  a  large  number  of  minerals. 
Briefly  expressed,  the  progressive  change,  approaching  the  line 
of  contact,  consists  in  a  gradual  decrease  in  the  proportional 
amount  of  silica  and  alkalies,  with  a  corresponding  increase  in 
iron  and  alumina,  this  being  accompanied  by  a  disappearance 
of  the  quartz  and  muscovite  and  the  development  of  biotite, 
sillimanite,  staurolite,  cyanite,  and  garnet,  as  above  mentioned. 
Where  limestones  abound,  they  have  become  bleached  and 
rendered  more  closely  crystalline,  while  a  variety  of  meta- 
morphic  minerals,  as  lime-bearing  pyroxenes,  hornblendes, 
zoisite,  sphene,  and  scapolite  have  been  developed. 


158 


METAMORPHIC    ROCKS 


A  common  form  of  metamorpliism  is  manifested  in  the  pro- 
duction of  a  quartzite  from  siliceous  sandstone.      This,  in  its 
simplest  form,  is  brought  about  by  a  secondary  deposit  of  silica 
about  the  original  rounded  granules  of  sand,  whereby  the  entire 
mass  is  converted  into  an  aggregate  of  quartz  crystals,  the  out- 
lines of  which  are  more  or  less  imperfect  through  mutual  in- 
terference in  process  of 
growth.  The  microscopic 
structure  of   a  quartzite 
of  this  nature  is  shown  in 
Fig.  13.     In  this  case  the 
original    rounded    gran- 
ules   are    readily  recog- 
nized from  the  fact  that 
not  merely  did  they  fre- 
quently    contain     small 
cavities    and   needle-like 
enclosures,  but  exteriorly 
they  were  covered  with  a 
thin  pellicle  of  iron  ox- 
ide, while  the  secondary 
deposit,  which   now  fills 
all  the  interspaces,  is  free 
from    enclosures    of    all 
kinds  and  quite  pellucid. 

In  many  quartzites  a  shearing  force  has  acted  a  prominent 
part,  whereby  the  granules  have  become  elongated  and  more  or 
less  pulverized  along  their  margins  by  the  friction  of  rubbing 
one  over  the  other.  In  such  cases  mica  and  other  secondary 
minerals  are  often  developed,  and  the  rock  passes  over  into  a 
mica  schist. 

Still  another  form  of  change,  or  metamorpliism,  is  that 
known  by  the  name  of  metasomatosis,  a  process  of  indefinite 
substitution  and  replacement.  Through  the  chemical  action 
of  percolating  solutions  certain  constituents  of  a  rock  may  be 
leached  out  and  replaced  by  others  in  indefinite  proportions. 
It  is  by  such  processes  that  have  originated  a  large  share  of 
the  serpentinous  rocks,  dolomites,  etc.  The  mineral  olivine, 
an  anhydrous  ferruginous  silicate  of  magnesia,  passes  over  into 
serpentine  by  a  simple  process  of  hydration,  and  a  more  or  less 
complete  change  of  its  combined  iron  from  the  ferrous  to  the 


FIG.  13.  —  Microstructure  of  quartzite,  showing 
secondary  deposit  of  silica  about  the  original 
quartz  grains. 


METASOMATOSIS  159 

ferric  state  ;  this  constituent  not  infrequently  separating  out 
during  the  process  of  change,  and  crystallizing  as  magnetite, 
or  remaining  as  an  amorphous  hematite  or  limonite.  Provided 
there  be  no  loss  in  silica,  this  change  in  the  olivine,  according 
to  T.  Sterry  Hunt,  must  be  accompanied  by  an  increase  of 
volume  amounting  to  some  33%.  Through  the  hydration  of 
eruptive  olivine-bearing  rocks,  or  rocks  rich  in  other  magnesian 
silicate  minerals,  have  originated  a  large  proportion  of  the  so- 
called  serpentines  and  verd-antique  marbles.  Many  serpentines 
and  serpentinous  limestones  are  derived  from  metamorphic 
rocks  rich  in  lime-magnesian  pyroxenes  or  amphiboles,  as  mala- 
colite  and  tremolite.  To  such  an  origin  are  to  be  referred 
such  serpentinous  limestones  as  those  of  Essex  County,  New 
York ;  Easton,  Pennsylvania,  and  Montville,  New  Jersey.  In 
the  last-named  instance  the  original  rock  was  a  coarsely  crys- 
talline dolomitic  limestone  containing  numerous  nodular  masses 
of  white  pyroxene  (malacolite).  Under  this  metasomatic  pro- 
cess they  yielded  up  their  calcium,  which  recrystallized  as 
calcium  carbonate  or  calcite,  while  the  silica  and  magnesia, 
combined  with  some  13  %  of  water,  remained  as  a  beautiful 
green  and  yellow  serpentine.  The  transformation  was  accom- 
panied by  a  considerable  increase  in  bulk,  whereby  the  exterior 
of  the  nodules,  pressed  against  the  rough  walls  of  the  enclos- 
ing rock,  became  scratched  and  polished  like  boulders  from  the 
glacial  drift,  or  the  entire  mass  even  took  on  a  platy,  schistose 
structure.  Figure  8,  from  a  specimen  in  the  National  Museum, 
illustrates  a  transitional  phase  of  this  change,  the  interior 
rounded  mass  of  a  gray  color  being  of  still  unaltered  pyroxene, 
while  the  dark  material  forming  the  exterior  shell,  or  travers- 
ing the  gray  in  fine  thread-like  veins,  is  the  secondary  ser- 
pentine. In  a  like  manner  in  all  probability  originated  the 
peculiar  structure  imitative  of  animal  organisms  known  as 
Eozoon  Canadense.1 

The  conversion  of  a  limestone  into  a  dolomite  is  believed  to 
have  been  brought  about  by  a  somewhat  similar  process.  Indeed 
it  is  doubtful  if  this  last-named  rock  is  ever  a  product  of  direct 
sedimentation  or  precipitation.  Although  sea-water  contains 

1  See  On  the  Serpentine  of  Montville,  New  Jersey,  Proc.  U.  S.  National 
Museum,  Vol.  XI,  1888 ;  Notes  on  the  Serpentinous  Rocks  of  Essex  County, 
New  York,  etc.,  ibid.,  Vol.  XII,  1889;  and  On  the  Ophiolite  of  Thurman, 
Warren  County,  New  York,  Am.  Jour,  of  Science,  Vol.  XXXVII,  1889. 


160  METAMORPHIC   ROCKS 

from  three  to  four  times  as  much  magnesia  as  lime,  evidence  is 
wanting  to  show  that  the  material  is  ever  secreted  in  appre- 
ciable quantities  by  marine  animals,  and  hence  the  sedimentary 
deposits,  resulting  from  the  accumulation  of  the  remains  of  these 
animals,  must  be  correspondingly  lacking  in  this  constituent.  It 
has  been  argued  by  Beaumont  and  others  that  through  a  process 
of  partial  molecular  replacement  (metasomatosis)  pre-existing 
limestones  were  converted  into  dolomites,  the  process  consisting 
in  the  replacement  of  every  other  molecule  of  calcium  carbonate 
by  one  of  the  magnesium  carbonate.  As  the  dolomite  molecule 
is  the  more  dense  of  the  two,  such  replacement,  in  any  given 
limestone  bed,  must  result  in  a  contraction  amounting  to  some 
12  J  °/0 .  Assuming  that  a  dolomitic  mass  resulting  in  this  way 
is  of  the  same  bulk  as  the  original  limestone,  this  shrinkage 
must  manifest  itself  in  the  production  of  interstitial  rifts  and 
cavities,  such  as  do  actually  occur  in  many  dolomitic  lime- 
stones, as  those  of  the  Ohio  Trenton  formations.  The  principal 
objection  to  this  theory  lies  in  the  difficulty  of  accounting  for 
the  large  amount  of  magnesia  in  solution ;  whence  its  source, 
etc.  The  same  objections  apparently  apply  to  the  explanation 
given  by  M.  C.  Klement.1  This  writer  describes  a  series  of 
experiments  in  which  solutions  of  sodium  chloride  and  magne- 
sium sulphate  were  made  to  act  upon  pulverized  calcite  and 
aragonite.  From  the  results  obtained,  he  concludes  that  dolo- 
mite is  formed  by  the  action  of  sea-water,  concentrated  in  en- 
closed basins  and  heated  by  the  sun,  on  the  aragonite  deposited 
by  marine  organisms,  in  such  a  way  that  a  mixture  of  carbon- 
ates of  calcium  and  of  magnesium  is  first  produced,  and  which 
is  subsequently  converted  into  dolomite. 

Still  another  theory  is  that  which  regards  the  dolomite  as  a 
residuary  product  formed  by  the  leaching  out  of  the  lime  car- 
bonate from  beds  of  impure,  slightly  magnesian  limestone, 
leaving  behind  the  less  soluble  magnesian  carbonate.  The 
amount  of  material  lost,  and  the  consequent  contraction  of  the 
original  beds,  must  necessarily  vary  with  their  purity ;  but  in 
any  case  where  the  residual  mass  has  reached  the  condition  of 
a  true  dolomite,  the  proportional  loss  must  have  been  enormous, 
since  in  no  cases  are  unaltered  sediments  known  to  contain 
more  than  4  or  5  %  of  magnesian  carbonate.  Although  on 
first  thought  this  theory  seems  the  more  plausible  of  the  two, 
1  Bull,  de  la  Societe  Geologique  de  Beige,  Tome  IX,  1895. 


HYDRO-METAMORPHISM  161 

it  is  apparently  rendered  invalid  by  the  presence  in  these  dolo- 
mites of  very  perfect  casts  of  fossils  which  have  undergone  no 
crushing  or  distortion  whatever,  and  which  tend  to  show  that 
the  beds  as  a  whole,  so  far  from  having  undergone  a  shrink- 
age of  95  %  and  upwards,  are  of  essentially  the  same  bulk  as 
when  laid  down.  The  subject  is  too  large  for  complete  dis- 
cussion here,  and  the  reader  is  referred  to  standard  works  on 
chemical  geology,  as  well  as  the  current  literature. 1 

Still  another  form  of  change  in  the  structure  and  mineral 
composition  of  a  rock  is  that  brought  about  through  the  action 
of  water  below  the  zone  of  oxidation  and  of  true  weathering. 
It  may  be  best  described  as  a  process  of  hydro-metamorphism, 
since  the  influence  of  water  is  paramount.  It  is  to  this  form 
of  metamorphism  that  is  due  the  production,  in  part,  of  secondary 
epidote,  chlorite,  sericite,  leucoxene,  kaolin  (?)  pyrite,  and  vari- 
ous zeolitic  compounds  from  pre-existing  minerals,  but  without 
in  any  way  changing  the  character  as  a  geological  body  of  the 
rock  mass  in  which  they  occur.  Such  changes  are  in  part  meta- 
somatic,  and  in  many  instances  are  rendered  more  intense  by 
dynamic  causes.  This  form  of  change  has,  unfortunately,  been 
too  frequently  confounded  with  weathering  and  decomposition.2 

Under  the  head  of  metamorphic,  then,  is  grouped  a  large 
series  of  rocks  which  have  been  changed  from  their  original 
condition  through  the  dynamical  and  chemical  agencies  above 
described,  and  which  may  have  been  in  part  of.  aqueous  and  in 
part  of  eruptive  origin.  Were  it  possible,  it  might  have  been 
better  to  describe  each'  class  of  these  rocks  together  with  the 
corresponding  igneous  or  aqueous  form  from  which  it  was  de- 
rived by  this  process  of  change,  or  metamorphism.  In  only 
too  many  cases,  however,  the  change  has  been  so  complete  as 
to  quite  obliterate  all  such  traces  of  the  original  character  as 
would  lead  to  safe  and  satisfactory  conclusions,  and  consistency 
demands  that  all  be  grouped  together. 

1  See  The  Magnesian  Series  of  the  Northwestern  States,  by  C.  W.  Hall  and 
F.  W.  Sardeson.     Bull.  Geol.  Soc.  of  America,  Vol.  VI,  1895,  p.  167. 

2  While  it  is  true  that  no  new  compound  can  be  formed  without  first  a  break- 
ing up,  or  decomposition,  of  those  already  existing,  still,  as  this  decomposition 
affects  only  the  individual  minerals,  and  not  the  integrity  of  the  rock  mass  as  a 
whole,  it  would  seem  preferable  to  include  such  changes  under  the  name  of 
alteration   and  metamorphism.     Weathering  it  certainly  is  not,  though  it  is 
essentially  the  form  of  change  which  Roth  (Allegemeine  u.  Chemische  Geologie, 
Vol.  I, -pp.  159-412)  has  designated  as  complex  weathering  (Complicirte   Vtr- 
witterung}. 


162  METAMORPHIC   ROCKS 

Accordingly  as  they  vary  in  structure,  we  may  divide  these 
metamorphic  rocks  into  two  general  groups  as  below:  1.  Strati- 
fied or  bedded;  2.  foliated  or  schistose. 


1.     STRATIFIED  OR  BEDDED 
(1)    THE   CRYSTALLINE   LIMESTONES   AND   DOLOMITES 

.  Here  are  included  the  metamorphosed  form  of  the  sedimen- 
tary rocks  described  on  p.  143. 

Mineral  Composition.  —  The  essential  constituent  of  the  crys- 
talline limestones  is  the  mineral  calcite.  The  common  acces- 
sories are  minerals  of  the  mica,  amphibole,  or  proxene  group, 
and  frequently  sphene,  tourmaline,  garnets,  vesuvianite,  apatite, 
pyrite,  graphite,  etc. 

Chemical  Composition.  —  As  may  be  inferred  from  the  mineral 
composition,  these  rocks,  when  pure,  consist  only  of  calcium 
carbonate.  They  are,  however,  rarely  if  ever  found  in  a  state 
of  absolute  purity,  but  show  more  or  less  magnesia,  alumina, 
and  other  constituents  of  the  accessory  minerals.  The  analyses 
given  on  pp.  146—47  will  serve  equally  well  here,  and  need  not 
be  repeated. 

Structure.  —  The  limestones  are  eminently  stratified  rocks, 
though  this  peculiarity  is  not  always  sufficiently  marked  to  be 
seen  in  the  hand  specimen.  The  purest  and  finest  crystalline 
varieties  often  show  a  granular  texture  like  that  of  loaf  sugar, 
and  hence  are  spoken  of  as  saccharoidal  limestones.  Statuary 
marble  is  a  good  illustration  of  this  type.  Under  the  micro- 
scope the  stone  is  shown  to  be  made  up  of  small  grains,  which, 
having  mutually  interfered  in  process  of  growth,  do  not  possess 
perfect  crystal  outlines,  but  are  rounded  and  irregular  in  out- 
line, as  shown  in  Fig.  14.  All  grades  of  textures  are  common, 
the  coarser  forms  sometimes  showing  individual  crystals  an  inch 
in  length.  Though  in  their  unchanged  conditions  highly  fossi- 
liferous  or  tufaceous,  these  structural  features  may  be  wholly 
or  in  part  obliterated  by  crystallization. 

Colors.  —  The  color  of  pure  limestone  is  snow-white,  as  seen 
in  statuary  marble.  Other  common  colors  are  pink  or  reddish, 
greenish,  blue-gray  through  all  shades  of  gray  to  black.  The 
pink  and  red  colors  are  due  to  iron  oxides,  the  greenish  as  a 


STRATIFIED  OR   BEDDED 


163 


rule  to  micaceous  minerals,  the  blue-gray  and  black  to  carbo- 
naceous matter. 

Geological  Age  and  Mode  of  Occurrence.  —  The  crystalline 
limestone  and  dolomites  are  but  the  metamorphosed  sedimentary 
deposits  such  as  have  al- 
ready been  described  on 
p.  143.  They  occur  asso- 
ciated with  rocks  of  all 
ages,  but  only  in  regions 
that  have  been  subjected 
to  disturbances  such  as  the 
folding  and  faulting  inci- 
dent to  mountain-making, 
or  the  heat  from  intruded 
igneous  rocks.  From  an 
economic  standpoint,  the 
rocks  of  this  group  are 
not  infrequently  of  great 
economic  value  for  struct- 
ural and  decorative  pur- 
poses. 

Classification  and  Nomenclature.  —  It  is  common  to  speak  of 
this  entire  group  of  rocks  as  simply  limestones,  though  many 
varietal  names  are  often  rather  indefinitely  applied.  The  name 
marble  is  given  to  any  calcareous  or  magnesian  rock  suffi- 
ciently beautiful  to  be  utilized  in  decorative  work.  Argilla- 
ceous and  siliceous  limestones  carry  clayey  matter  and  sand. 
Dolomite  (so  named  after  the  French  geologist  Dolomieu) 
consists  of  45.50%  carbonate  of  magnesia  and  54.40%  car- 
bonate of  lime,  as  already  noted.  The  names  ophiolite  and 
opliicalcite  are  popularly  applied  to  stones  consisting  of  a 
granular  aggregate  of  calcite  and  serpentine,  such  as  occur 
in  Essex  County,  New  York,  and  are  used  as  marbles.  The 
so-called  Eozoon  Canadenses,  a  supposed  fossil  rhizopod,  belongs 
here.  The  serpentinous  matter  in  such  cases  originates  from  a 
non-aluminous  pyroxene  by  a  process  of  hydration,  as  already 
explained. 


FIG.  14.  —  Microstructure  of  crystalline  lime- 
stone (marble). 


164  METAMORPHIC   ROCKS 

2.     FOLIATED  OR  SCHISTOSE 

(1)   THE   GNEISSES 

Gneiss,  from  the  German  Gf-neis,  a  term  used  by  the  miners 
of  Saxony  to  designate  the  country  rock  in  which  occur  the 
ore  deposits  of  the  Erzgebirge.  The  word  is  pronounced  as 
though  spelled  nice. 

Mineral  and  Chemical  Composition.  —  The  composition  of  the 
gneisses  is  essentially  the  same  as  that  of  the  granites,  from 
which  they  differ  only  in  structure  and  origin.  They,  how- 
ever, present  a  greater  variety  and  abundance  of  accessory 
minerals,  chief  among  which  may  be  mentioned  (besides  those 
of  the  mica,  hornblende,  or  pyroxene  group)  garnet,  tour- 
maline, beryl,  sphene,  apatite,  zircon,  cordierite,  pyrite,  and 
graphite. 

Structure.  —  Structurally  the  gneisses  are  holocrystalline 
granular  rocks,  as  are  the  granites,  but  differ  in  that  the 
various  constituents  are  arranged  in  approximately  parallel 
bands  or  layers,  as  shown  in  PI.  13. 

In  width  and  texture  these  bands  vary  indefinitely.  It  is 
common  to  find  bands  of  coarsely  crystalline  quartz  several 
inches  in  width,  alternating  with  others  of  feldspar,  or  feld- 
spar, quartz,  and  mica,  or  hornblende.  A  lenticular  structure 
is  common,  produced  by  lens-shaped  aggregates  of  quartz  or 
feldspar,  about  and  around  which  are  bent  the  hornblendes  or 
mica  laminae.  The  rocks  vary  from  finely  and  evenly  fissile 
through  all  grades  of  coarseness,  and  become  at  times  so  mas- 
sive as  to  be  indistinguishable  in  the  hand  specimens  from 
granites.  The  causes  of  the  foliated  structure  are  mentioned 
below. 

Colors.  —  Like  the  granites,  they  are  all  shades  of  gray, 
greenish,  pink,  or  red. 

Geological  Age  and  Mode  of  Occurrence.  —  The  true  gneisses  are 
among  the  oldest  crystalline  rocks,  and  have  been  considered  by 
many  geologists  as  representing  "portions  of  the  primeval  crust 
of  the  globe,  traces  of  the  surface  that  first  congealed  upon  the 
molten  nucleus."  By  others  they  are  regarded  as  metamor- 
phosed sedimentary  deposits  resulting  from  the  breaking  down 
of  still  older  rocks,  and  may  not  in  themselves,  therefore,  be  con- 
fined to  any  particular  geological  horizon.  They  are  in  large 


PLATE   13 


i  mm 


FIG.  1.  Banded  gneiss. 


FIG.  2.   Foliated  gneiss. 


THE   GNEISSES  165 

part,  however,  indisputably  the  oldest  known  rocks,  lying  be- 
neath or  being  cut  by  all  rocks  of  later  formation  or  injection. 
The  origin  of  the  gneisses,  as  above  suggested,  is  in  many 
cases  somewhat  obscure,  the  banded  or  foliated  structure  being 
considered  by  some  as  representing  the  original  bedding  of  the 
sediments,  the  different  bands  representing  layers  of  varying 
composition.  This  structure  is  now,  however,  considered  to  be 
due  to  mechanical  causes,  and  in  no  way  dependent  upon  origi- 
nal stratification.  The  name,  as  commonly  used,  is  made  to  in- 
clude rocks  of  widely  different  structure,  and  which  are  beyond 
doubt  in  part  sedimentary  and  in  part  eruptive,  but  in  all  cases 
altered  from  their  original  conditions. 


FIG.  15.  —  Microstructure  of  gneiss,  showing  at  the  points  a  broken  feldspars. 

This  alteration,  it  should  be  stated,  has  been  brought  about 
not  by  heat  and  crystallization  alone,  but  in  many  cases  by 
processes  of  squeezing,  crumpling,  and  folding  so  complex  as 
almost  to  warrant  the  application  of  the  term  kneading  thereto. 
It  is  even  possible  to  conceive  that  some  of  them  may  be  origi- 
nal massive  or  foliated  rocks  into  which  eruptive  materials  have 
since  been  injected  along  lines  of  foliation  or  of  weakness  due  to 
shearing,  and  the  entire  mass  again  submitted  to  such  a  knead- 
ing as  to  render  it  practically  impossible  to  now  decide  what 
are  portions  of  the  original  rock  and  what  of  the  subsequently 
injected. 


166 


METAMORPHIC   ROCKS 


The  •  close  chemical  relationship  which  may  exist  between 
clastic,  metamorphic,  and  eruptive  rocks  is  shown  in  the  selected 
series  of  analyses  here  given. 


CONSTITUENTS 

GRANITE 

GNEISS 

8 

N 
Jz, 

o 

SANDSTONE 

M 

3 

«j 

OQ 

I 

OQ 

DISINTE- 
GRATED 
GRANITE 

I 

II 

III 

IV 

v 

VI 

VII 

Silica  (Si02) 

68  18 

% 
61.96 

69.24 

% 
69  94 

% 

61.91 

% 

60.32 

65.69 

Titanium  oxide  (Ti02)  . 

1.66 

Not  det 

0.31 

Alumina  (A1203)      .... 
Ferric  oxide  (Fe2Os)     .     .     . 
Ferrous  oxide  (FeO) 

16.20 
4.10 

19.73 
4.60 

14.85 
2.62 

13.15 

2.48 

21.73 
4.73 

23.10 
7.05 

15.23 
4.39 

Ferrous  sulphide  (FeS2) 

4.33 

Manganese  oxide  (MnO)    .     . 
Lime  (CaO) 

1  75 

Trace 
0  35 

0.45 
2  10 

0.70 

3.08 

0  09 

.... 

Not  det. 
2  63 

Magnesia  (MgO)  .     . 

0.48 

1.81 

096 

Trace 

0.59 

0.87 

2.64 

Soda  (Na20)    

2.88 

0.79 

4.30 

5.43 

0.25 

0.49 

2.12 

Potash  (K20) 

6  48 

2  50 

4  33 

3  30 

3  16 

3  83 

2  00 

Ignition 

1  82 

0  70 

1  01 

7  43 

4  08 

4.70 

100.07 

99.55 

99.56 

99.10 

99.89 

99.74 

99.71 

I.  Granite :  Syene,  Egypt.  II.  Gneiss :  St.  Jean  de  Matha,  Province  of  Que- 
bec, Canada.  III.  Gneiss :  Trembling  Mountain,  Province  of  Quebec,  Canada. 
IV.  Sandstone :  Portland,  Connecticut.  Y.  Shale :  England.  VI.  Slate  :  Lan- 
caster County,  Pennsylvania.  VII.  Disintegrated  granite :  District  of  Columbia. 

Figures  1  and  2  on  PI.  13  shows  two  rather  extreme  types  of 
these  gneissoid  rocks.  Figure  1  is  that  of  a  banded  gneiss  from 
Madison  County,  Montana,  and  which,  so  far  as  we  know,  may 
be  an  altered  sedimentary  rock.  In  Fig.  2  of  the  same  plate 
is  shown  a  foliation  rather  than  a  banded  rock,  and  whatever 
may  have  been  its  origin,  it  undoubtedly  owes  its  foliated 
structure  to  dynamic  agencies.  The  effect  of  the  shearing 
force  whereby  the  foliation  was  produced  is  evident  in  the 
figure,  even  to  the  unaided  eye,  to  the  left  and  just  above  the 
centre,  where  an  elongated  feldspar  is  seen  broken  transversely 
in  four  pieces.  The  same  features  are  brought  out  even  more 
plainly  in  Fig.  15,  which  shows  the  structure  of  this  same  gneiss 
as  seen  under  the  microscope. 

As  in  the  present  state  of  our  knowledge  it  is  in  most  cases 
impossible  to  separate  what  may  be  true  metamorphosed  sedi- 


GNEISS 


167 


mentary  rocks  from  those  in  which  the  foliated  or  banded 
structure  is  in  no  way  connected  with  bedding  and  which  may 
or  may  not  be  altered  eruptives,  all  are  grouped  together  here. 

Classification  and  Nomenclature.  —  The  varietal  distinctions 
are  based  upon  the  character  of  the  prevailing  accessory  min- 
eral, as  in  the  granites,  forming  a  parallel  series.  We  thus 
have  biotite  gneiss,  muscovite  gneiss,  biotite-muscovite  gneiss,  horn- 
blende gneiss,  etc.  Rarely  the  mineral  cordierite  occurs  in  suffi- 
cient abundance  to  become  a  characterizing  accessory.  Such 
forms  occur  in  Gilford  County,  Connecticut,  and  in  Saxony. 

The  name  granulite  or  leptynite  is  applied  to  a  banded  quartz- 
feldspar  rock,  the  constituents  of  which  occur  in  the  form  of 
small  grains  and  show  under  the  microscope  a  mosaic  structure. 
The  Saxon  granulites  are  regarded  by  Lehman  as  eruptive 
rocks  altered  by  pressure.  Halleflinta  is  a  Swedish  name  for 
a  rock  resembling  in  most  respects  the  eruptive  felsites  or 
quartz  porphyries  already  described.  Such,  however,  show  a 
banded  structure  and  are,  as  a  rule,  regarded  as  metamorphic 
rocks.  PorpJiyroid  is  also  a  felsitic  rock  with  a  more  or  less 
schistose  structure,  and  with  porphyritic  feldspar  or  quartzes. 
Such  have  been  described  from  the  Ardennes,  France. 

GNEISS 


ANALOGOUS  MASSIVE  TYPE 

OF  IGNEOUS  ORIGIN 

ORIGIN  UNKNOWN 

Granite  : 
Biotite  granite      .     .     . 
Hornblende  granite  .     . 

Syenite  : 
Hornblende  syenite  .     . 

Mica  syenite     .... 
Pyroxene  syenite      .     . 
Diorite  : 
Mica  diorite     .... 
Gabbro   . 

Granite  gneiss  : 
Biotite  granite  gneiss  . 
Hornblende  granite     \ 
gneiss     .     .     .     .    / 
Syenite  gneiss  : 
Hornblende  syenite     1 
gneiss     .     .     .     .    / 
Mica  syenite  gneiss.     . 
Pyroxene  syenite  gneiss 
Diorite  gneiss  : 
Mica  diorite  gneiss  .     . 
Gabbro  gneiss    .... 

Granitic  gneiss  : 
Biotite  granitic  gneiss. 
Hornblende  granitic 
gneiss. 
Syenitic  gneiss  : 
Hornblende  syenitic 
gneiss. 
Mica  syenitic  gneiss. 
Augite  syenitic  gneiss. 
Dioritic  gneiss  : 
Mica  dioritic  gneiss. 
Gabbroic  gneiss,  or  gab- 

Pyroxenite  

Pyroxenite  gneiss  .     .     . 

brie  gneiss. 
Pyroxenitic  gneiss. 

Inasmuch  as  the  structure  characteristic  of  gneisses  is  found 
developed  in  rocks  of  diverse  types,  many  petrologists  now  use 


168  METAMORPHIC   KOCKS 

the  term  in  an  almost  wholly  structural  sense,  as  in  itself  non- 
committal as  to  composition  or  origin,  but  merely  designating  a 
rock  of  foliated  or  schistose  structure.  C.  H.  Gordon  has  pro- 
posed1 a  scheme  of  classification  of  gneissoid  rocks  as  above, 
and  which  has  much  in  its  favor. 


(2)   THE   CRYSTALLINE   SCHISTS 

Under  this  head  are  grouped  a  large  and  extremely  variable 
series  of  rocks,  differing  from  the  gneisses  mainly  in  the  lack  of 
feldspar  as  an  essential  constituent.  They  consist,  therefore, 
essentially  of  granular  quartz,  with  one  or  more  minerals  of  the 
mica,  chlorite,  talc,  amphibole,  or  pyroxene  group.  In  acces- 
sory minerals  the  schists  are  particularly  rich.  The  more 
common  of  these  are  feldspar,  garnet,  cyanite,  staurolite, 
tourmaline,  epidote,  rutile,  magnetite,  menaccanite,  and  pyrite. 
Through  an  increase  in  the  proportional  amount  of  feldspar  the 
schists  pass  into  the  gneisses,  and  through  a  decrease  in  mica, 
hornblende,  or  whatever  may  be  the  characterizing  mineral, 
into  the  quartz  schists,  in  which  quartz  alone  is  the  essential 
constituent.  Occasional  forms  are  met  with  quite  lacking 
in  quartz  and  other  accessory  minerals  and  consisting  only  of 
schistose  aggregates  of  minerals  of  a  single  species,  as  is  the 
case  with  the  pyrophyllite  schists  (or,  more  properly,  schistose 
pyrophyllites)  from  North  Carolina,  talcose  schists,  and  with 
the  more  massive  "soapstones." 

The  rocks  of  this  group  are  characterized  as  a  whole  by  a 
pronounced  schistose  structure,  due  to  the  parallel  arrangement 
of  the  various  constituents,  this  structure  being  most  pro- 
nounced in  those  varieties  in  which  mica  is  the  predominating 
accessory  mineral.  They  are  ordinarily  considered  as  having 
originated  from  the  crystallization  of  sediments,  and  in  many 
cases  'the  microscope  still  reveals  existing  "  traces  of  the  origi- 
nal grains  of  quartz  sand  and  other  sedimentary  particles  of 
which  the  rocks  at  first  consisted."  Like  the  gneisses,  they 
are  in  part,  however,  mechanically  deformed  massive  rocks  and 
their  schistosity  in  no  way  relates  to  true  bedding,  as  has  been 
already  noted  (p.  156). 

The  varietal  names  given  are  dependent  mainly  upon  the 
character  of  the  prevailing  ferro-magnesian  silicate.  We  thus 

1  Bull.  Geol.  Soc.  of  America,  Vol.  VII,  p.  122. 


THE   CRYSTALLINE   SCHISTS 


169 


have  mica  schists,  chlorite  schists,  talc  schists,  hornblende,  actinol- 
ite,  glaucophane  schists,  etc.  The  term  slate  was  originally 
applied  to  these  and  other  types  of  rocks  of  schistose  or  fis- 
sile character.  In  the  arrangement  here  adopted  this  term  is 
restricted  to  the  argillaceous  fragmental  or  semi-crystalline 
and  foliated  rocks  next  to  be  described. 

Of  the  above-mentioned  varieties  the  mica  schists  are  the 
most  common  and  widely  distributed,  the  mica  being  in  some 
cases  biotite,  in  others  muscovite,  or  perhaps  a  mixture  of 
the  two.  The  principal  accessories  sufficiently  developed  to 
be  conspicuous  are  staurolites,  chiastolites,  garnets,  and  tour- 
malines. In  the  sericite  schists  the  hydrous  mica  sericite 
prevails;  paragonite  schist  carries  the  hydrous  sodium-mica  par- 
agonite  ;  ottr elite  schist  carries  the  accessory  mineral  ottrelite. 

The  name  phyllite  is  used  by  German  petrographers  to  desig- 
nate a  micaceous  semi-crystalline  rock  standing  intermediate 
between  the  true  schists 
and  clay  slates.  Quart- 
zite  is  a  more  or  less 
schistose  or  banded  rock 
consisting  essentially  of 
crystalline  granules  of 
quartz.  Such  originate 
from  the  induration  of  si- 
liceous sandstones.  This 
induration  is  brought 
about  through  a  deposi- 
tion of  crystalline  silica 
in  the  form  of  a  bind- 
ing material  or  cement 
around  each  of  the  sand 
particles  of  which  the 
stone  is  composed.  Each 
of  these  granules  then  forms  the  nucleus  of  a  more  or  less  per- 
fectly outlined  quartz  crystal.  This  structure  is  shown  in  Fig. 
16,  drawn  from  a  thin  section  of  a  Potsdam  quartzite  from 
St.  Lawrence  County,  New  York.  The  rounded,  more  or  less 
shaded,  portions  represent  the  original  grains  of  quartz  sand, 
and  the  clear,  colorless,  interstitial  portions  the  secondary  silica. 

The  quartzites  consist,  as  a  rule,  only  of  silica,  or  silica 
colored  brown  and  red  by  iron  oxides.  At  times  a  greenish 


FIG.  16.  —  Microstructure  of  quartzite. 


170  METAMORPHIC    ROCKS 

tinge  is  imparted  through  the  development  of  chloritic  minerals; 
accessory  minerals  are  not,  as  a  rule,  abundant. 

Among  the  hornblende  schists  there  are  but  few  needing 
especial  attention.  These  are,  as  a  rule,  less  finely  schistose 
than  are  the  mica-bearing  schists,  owing  to  the  fact  that  the 
mineral  hornblende  itself  has  not  a  platy  structure.  The  glau- 
cophane  schists  are  perhaps  the  least  abundant  of  the  hornblendic 
varieties.  Such  have  been  described  from  the  Isle  of  Syra,  in 
the  Mediterranean  Sea,  Switzerland,  Wales,  and  Italy ;  a  more 
massive  form,  probably  an  altered  eruptive,  is  found  near  the 
mouth  of  Sulphur  Creek,  Sonoma  County,  California.  Am- 
phibolite  is  the  name  given  to  an  extremely  tough  and  often 
massive  rock  of  obscure  origin,  and  consisting  essentially  of 
the  mineral  amphibole  or  hornblende.  In  some  instances  the 
varieties  of  amphibole,  actinolite,  and  tremolite  take  the  place 
of  the  common  hornblende.  The  tremolite  rock  may  undergo 
alteration  into  serpentine  under  proper  conditions.  Edogite  is 
a  tough,  massive,  or  slightly  schistose  rock,  consisting  of  the 
grass-green  variety  of  pyroxene,  omphacite,  and  small  red  gar- 
nets, with  which  are  frequently  associated  bluish  kyanite,  green 
hornblende  (smaragdite),  and  white  mica.  Grarnet  rock,  or 
garnetite,  is  a  crystalline  granular  aggregate  of  garnets  with 
black  mica,  hornblende,  quartz,  and  magnetite.  Kinzigkite  is 
a  somewhat  similar,  though  fine-grained  and  compact,  rock 
consisting  of  garnets,  plagioclase  feldspar,  and  black  mica, 
and  which  is  found  in  Kinzig  and  the  Odenwald. 

Many  of  the  rocks  of  this  group  are  but  products  of  dynamic 
or  contact  metamorphism,  as  is  the  case  with  many  of  the 
chiastolite  and  argillaceous  schists  or  roofing  slates.  Rocks  of 
the  latter  group  pass  by  insensible  gradations  into  clastic  ar- 
gillites.  They  owe  their  cleavable  property  to  shearing,  as 
already  explained.  Under  the  microscope  these  rocks  are 
found  to  be  quite  variable.  Hawes  describes  clay  slate  from 
Littleton,  New  Hampshire,  as  consisting  of  a  mixture  of  quartz 
and  feldspar,  in  particles  as  fine  as  dust.  They  contained  also 
amorphous  carbonaceous  matter  and  little  needles  of  a  mineral 
assumed  to  be  mica.  A  slate  from  Hanover,  in  the  same  state, 
contained  garnets  and  staurolites.  Wichman  found  slates  from 
Lake  Superior  to  consist  of  a  colorless,  isotropic  ground-mass 
carrying  quartz  and  feldspar  particles,  scales  of  iron  oxide,  car- 
bonaceous matter,  minute  tourmalines,  and  mica  fragments. 


THE   CRYSTALLINE   SCHISTS 


171 


The  red  slates  of  New  York  state  are  composed  of  an  impal- 
pable red,  dust-like  ground-mass,  carrying  grains  of  quartz 
and  feldspar,  all  arranged  with  their  longer  axes  parallel  to 
the  plane  of  schistosity.  These  can  scarcely  be  considered  as 
other  than  clastic  rocks,  the  dynamic  action  not  having  been 
sufficient  to  produce  crystallization  in  more  than  incipient 
stages.  In  this  case  the  plane  of  schistosity  is  very  nearly 
parallel  with  that  of  bedding,  but  in  many  cases,  as  in  the 
roofing  slates  of  Pennsylvania,  the  schistose  structure  is  devel- 
oped at  a  very  considerable,  though  ever-varying,  angle  with 
the  bedding.  In  such  cases  the  true  bedding  plane  is  often 
determinable  only  by  the  dark  bands,  or  ribbons,  by  which  the 
split  slates  are  traversed. 

Chemical  Composition. — As  may  be  readily  imagined,  the 
schists  vary  almost  indefinitely  in  composition,  approximating 
pure  quartzite  on  the  one  hand  and  the  gneisses  on.  the  other. 
The  table  given  below  is  intended  to  show  the  composition  of 
a  few  characteristic  types  only.  All  gradations,  from  the  most 
acid  of  quartzites  to  the  most  basic  of  the  amphibolites,  may 
readily  be  found. 


CONSTITUENTS 

I 

II 

in 

IV 

v 

VI 

Silica  (Si02)        .     . 

82  38  °/n 

40.00  % 

52  39  % 

49.18% 

50  81  % 

97  1  % 

Alumina  (A1203)     .     .     . 

11.84 

23.65 

16.33 

15.09 

4.53 

1.39 

Ferric  oxide  (Fe203)    .     . 



8.07 

1.64 

12.90 

3.52 

1.25 

Ferrous  oxide  (FeO)   .     . 

2.28 

.... 

1.44 

4.26 

.... 

Lime  (CaO)    

.... 

0.63 

8.76 

10.59 

0.18 

Magnesia  (MgO)      .     .    . 

1.00 

0.94 

4.70 

5.22 

31.55 

0.13 

Potash  (K20)      .... 

0.83 

9.11 

1.42 

1.51 

.... 

Soda  (Na20) 

0  38 

1  75 

2  59 

3.64 

Ignition  . 

0  77 

3  41 

0  17 

1.87 

4.42 

I.  Mica  schist :  Monte  Rosa,  Switzerland.  II.  Sericite  schist :  Wisconsin. 
III.  Hornblende  schist:  Grand  Rapids,  Wisconsin.  IV.  Chlorite  schist :  Klippe, 
Sweden.  V.  Talc  schist:  Gastein,  Austria.  VI.  Quartzite:  Chickies  Station, 
Pennsylvania.  All  analyses  quoted  from  J.  F.  Kemp's  Lecture  Notes  on  Rocks. 


PART   III 


THE  WEATHERING  OP  ROCKS 

"  In  the  economy  of  the  world,  I  can  find  no  traces  of  a  beginning,  no  prospect  of 
an  end."  —  HUTTON. 

THE  stability  of  chemical  compounds  is  governed  by  prevail- 
ing conditions.  A  form  of  combination  stable  under  conditions 
existing  to-day  may,  under  those  of  to-morrow,  become  impos- 
sible. As  was  suggested  in  the  introductory  chapter,  the  con- 
ditions under  which  the  more  superficial  portions  of  the  earth's 
crust  exist  are  ever  changing,  and  as  a  result  old  compounds 
are  broken  up  and  new  continually  formed.  All  over  the  earth 
rocks  laid  down  as  sediments  on  oceanic  floors  have  been 
folded,  faulted,  and  pushed  out  of  place  until  brought  under 
influences  as  different  from  those  under  which  they  were  formed 
as  it  is  possible  to  conceive.  Molten  magmas  cooling  suddenly 
on  the  immediate  surface  formed  compounds  in  which  mere  loss 
of  heat  was  the  controlling  factor,  but  which  time  proves  to  be 
unstable.  Slow  cooling,  deep-seated  magmas  have  been,  and 
are  being,  continually  exposed  by  denudation,  and  thus  brought 
under  new  influences  and  environments.  Hence  a  constant 
readjustment  is  everywhere  going  on,  which,  as  we  shall  see,  is 
manifold  in  its  physical  manifestations.  As  where  an  entire 
building  is  razed  to  the  ground,  and  another  of  quite  different 
architectural  features  constructed  from  the  old  materials  ;  or 
again,  where,  without  change  of  general  plan,  old  timbers  are 
here  and  there  replaced  by  new,  so  here  we  have  at  work  a 
series  of  processes  in  part  seemingly  destructive  and  in  part 
constructive,  but  all  tending  toward  one  end. 

The  firm  and  everlasting  hills  we  must  learn  to  regard  as 
neither  firm  nor  everlasting.  Whole  mountain  chains  of  the 
geological  yesterday  have  disappeared  from  view,  and  as  with 

172 


PLATE    14 


Weathered  granite,  District  of  Columbia. 


THE   WEATHERING   OF   ROCKS  173 

the  ancient  cities  of  the  East,  we  read  their  histories  only  in 
their  ruins.  Yet,  in  all  this  seemingly  destructive  process  of 
breaking  down,  decomposition,  and  erosion,  there  is  traceable 
the  one  underlying  principle  of  transformation  from  the  un- 
stable toward  that  which  is  to-day  more  stable.  Nothing  is 
lost  or  wasted:  It  is  a  change  which  began  with  the  beginning 
of  matter  ;  which  will  end  only  with  the  blotting  out  of  matter 
itself.  There  are  no  traces  of  a  beginning,  there  is  no  prospect 
of  an  end. 


I.    THE   PRINCIPLES    INVOLVED    IN    ROCK- 
WEATHERING 

The  processes  involved  in  this  readjustment  from  unstable 
to  stable  compounds,  as  above  outlined,  and  of  incidental 
soil  formation,  are  in  part  physical  and  in  part  chemical  in 
their  nature ;  they  operate  under  ever- varying  conditions,  and 
through  processes  at  times  simple,  or  again  complex.  What 
these  processes  are,  and  how  they  operate,  it  must  be  our 
purpose  to  now  consider. 

It  may  be  said  at  the  outset,  that  whatever  the  forces  en- 
gaged, they  are,  with  a  few  isolated  exceptions,  superficial,  — 
they  work  from  without  downwards.  However  much  they  may 
have  accomplished  since  the  first  rock  masses  appeared  above 
the  primeval  ocean,  in  no  case  can  the  actual  amount  of  debris 
in  situ  have  formed  at  one  time  more  than  a  scarcely  appreciable 
film  over  the  underlying  and  unchanged  material.  The  decom- 
posing forces  early  lose  their  active  principles  and  become  quite 
inert  at  depths  comparatively  insignificant.  It  is  only  where 
through  erosion  the  results  of  the  disintegration  are  gradually 
removed,  that  the  processes  have  gone  on  to  such  an  extent  as 
to  pexhaps  quite  obliterate  thousands  of  feet  of  strata  or  of 
massive  rock,  and  furnished  the  necessary  debris  for  the  vast 
thicknesses  of  sandstone,  slate,  and  shale  which  characterize  the 
more  modern  horizons.  In  certain  isolated  cases,  it  is  true, 
ascending  steam  and  heated  waters,  arising  from  depths  un- 
known, have  been  instrumental  in  promoting  decomposition,  as 
is  well  illustrated  in  the  areas  of  decomposed  rhyolites  in  the 
Yellowstone  National  Park.  Nevertheless,  it  is  to  the  almost 
incalculably  slow  process  of  superficial  weathering  that  we  owe 


174      THE   PRINCIPLES   INVOLVED   IN  ROCK- WEATHERING 

a  very  large  share  of  the  apparent  rock  decomposition  and  inci- 
dental soil  formation.1 

This  transformation,  as  already  noted,  takes  place  through 
processes  that  may  be  simple,  or  again  complex.  It  is  but 
rarely  that  one,  alone,  prevails  for  any  length  of  time,  and  as  a 
rule  several  or  many  go  merrily  on  together.  Were  it  possi- 
ble, it  might  be  well  to  consider  briefly  each  of  these  in  its  turn 
and  by  itself.  From  the  fact,  however,  as  above  stated,  that 
any  one,  either  physical  or  chemical,  rarely  goes  on  alone,  it  is 
thought  best  to  treat  the  subject  as  below,  and  describe  in  more 
or  less  detail  the  action,  first,  of  the  atmosphere,  second,  of  water, 
in  both  the  solid  and  liquid  form,  and  third,  that  of  plant  and 
animal  life,  finally  considering  the  combined  action  of  all  these 
forces,  as  manifested  on  the  various  types  of  rock  which  go  to 
make  up  the  earth's  crust. 

So  striking  a  phenomenon  as  the  breaking  down,  or  degenera- 
tion as  we  may  call  it,  of  a  mass  of  firm  rock,  naturally  did  not 
escape  the  observation  of  the  earlier  workers  in  this  and  allied 
branches  of  science,  and  the  older  literature  from  the  time  of 
Hutton  contains  numerous  references  to  it,  though  the  full 
significance  of  atmospheric  agencies  in  bringing  about  the 
results,  seems  not  at  first  to  have  been  fully  realized. 

The  exciting  cause  of  the  degeneration,  particularly  in  warm 
latitudes,  where  phenomena  of  this  nature  are,  as  a  rule,  more 
apparent,  has  been  a  matter  of  some  speculation,  and  at  the  out- 
set it  may  be  well  to  indicate  in  brief  their  tendencies. 

1  The  term  weathering,  as  here  used,  is  applied  only  to  those  superficial 
changes  in  a  rock  mass  brought  about  through  atmospheric  agencies,  and  result- 
ing in  a  more  or  less  complete  destruction  of  the  rock  as  a  geological  body,  as 
where  granitic  rocks  are  resolved  into  sand,  and  kaolinic  material,  with  liberation 
of  carbonates  of  the  alkalies  and  of  lime,  and  oxides  of  iron.  It  does  not  include 
those  deeper-seated  changes  —  changes  taking  place  below  the  zone  of  oxidation 
arid  which  result  mainly  in  hydration  and  the  production,  it  may  be,  of  new 
mineral  species,  as  chlorite,  sericite,  zeolites,  etc.,  but  during  which  the  rock 
mass  as  a  whole  retains  its  individuality  and  geological  identity.  The  distinction 
is  not  one  that  has  been  sharply  insisted  upon,  and  indeed  geologists  and  petrolo- 
gists  as  a  rule  have  been  extremely  careless  in  their  use  of  such  terms  as  altera- 
tion, decomposition,  and  weathering.  The  distinction  drawn  here  is  essentially 
that  made  by  Roth  (Allegemeine  u.  Chemische  Geologie),  between  Verwitterung 
and  Complicirte  Verwitterung.  For  reasons  above  stated  and  others  given  on 
p.  161,  it  seems  best  to  limit  the  terms  weathering  and  decomposition  to  processes 
involving  the  destruction  of  the  rock  mass  as  a  geological  body,  and  to  designate 
the  purely  mineralogical  deeper-seated  changes  as  alteration,  which  may  or  may 
not  be  due  wholly  to  hydrometamorphism. 


OPINIONS   OF   EARLY   WORKERS  175 

Fournet,  as  quoted  elsewhere,  writing  as  early  as  1833,  in- 
sisted upon  the  efficacy  of  water  containing  carbonic  acid  in 
promoting  the  decomposition  of  igneous  rocks,  while  Brogniart, 
writing  with  particular  reference  to  feldspathic  decomposition 
and  the  origin  of  kaolin,  laid  great  stress  on  the  acceleration 
of  the  ordinary  process  of  decay  through  the  electric  currents 
resulting  from  the  contact  of  heterogeneous  rock  masses.  Dar- 
win 1  believed  the  extensive  decomposition  observed  by  him  in 
Brazil,  to  have  taken  place  under  the  sea,  and  before  the  present 
valleys  were  excavated.  Hartt 2  gave  it  as  his  opinion  that  the 
decomposition  was  due  to  the  action  of  warm  rain  water  soaking 
through  the  rock,  and  carrying  with  it  carbonic  acid  derived 
not  only  from  the  air,  but  from  the  vegetation  decaying  in  the 
soil  as  well,  together  with  organic  acids,  nitrate  of  ammonium, 
etc.  Further,  that  the  decomposition  had  gone  on  only  in  re- 
gions once  covered  by  forests.  Heusser  and  Claraz  3  suggested 
that  the  decomposition  was  brought  about  through  the  influence 
of  nitric  acid.  They  say  "  it  is  without  doubt  determined  by 
the  violence  and  frequency  of  the  tropical  rains,  and  by  the  dis- 
solving action  of  water,  which  increases  with  the  temperature. 
It  is  necessary  to  observe,  moreover,  that  this  water  contains 
some  nitric  acid,  on  account  of  the  thunder  storms  which  follow 
each  other  with  great  regularity  during  many  months  of  the 
year." 

Belt,4  in  discussing  the  extensive  decomposition  observed  by 
him  in  Nicaragua,  says :  "  This  decomposition  of  the  rocks 
near  the  surface  prevails  in  many  parts  of  tropical  America, 
and  is  principally,  if  not  always,  confined  to  the  forest  regions. 
It  has  been  ascribed,  and  probably  with  reason,  to  the  percola- 
tion through  the  rocks  of  rain  water  charged  with  a  little  acid 
from  the  decomposing  vegetation." 

The  elder  Agassiz  laid  much  stress  on  the  decomposing  effects 
of  the  hot  water  from  rainfall,5  while  Mills  and  Branner,6  in 
addition,  attributed  no  insignificant  amount  of  the  decomposi- 
tion to  the  action  of  decomposing  organic  matter  carried  into 

1  Geological  Observations,  p.  417. 

2  Phys.  Geog.  and  Geol.  of  Brazil. 

»  Ann.  des  Mines,  5th  series,  17,  1860,  p.  291. 
4  The  Naturalist  in  Nicaragua,  1874. 
6  Journey  in  Brazil,  p.  89. 

3  Bull.  Geol.  Soc.  of  America,  Vol.  VII,  1896. 


176       THE   PRINCIPLES   INVOLVED   IN    ROCK-WEATHERING 

the  ground  by  ants,  and  also  to  the  acid  secretions  of  the  ants 
themselves. 

The  chemical  changes  involved  in  the  process  of  decompo- 
sition received  attention  from  several  of  the  earlier  workers, 
among  whom  the  names  of  Berthier,  J.  G.  Forschammer, 
Brogniart,  Gustav  Bischof,  and  Ebelmen  stand  out  in  greater 
prominence.  More  recently  the  name  of  S terry  Hunt  becomes 
conspicuous,  while  the  purely  geological  side  of  the  question 
has  been  ably  set  forth  in  numerous  papers  by  L.  Agassiz,  R. 
Pumpelly,  N.  S.  Shaler,  O.  A.  Derby,  R.  Irving,  J.  C.  Branner, 
and  others,  to  whom  reference  is  frequently  made  in  these  pages. 


1.     ACTION  OF  THE  ATMOSPHERE 

Atmospheric  air,  as  is  well  known,  consists  in  its  normal  state 
of  a  mechanical  admixture  of  free  nitrogen  and  oxygen  in  the 
proportion  of  four  volumes  of  the  former  to  one  of  the  latter. 
In  addition  are  small  and  comparatively  insignificant  amounts 
of  various  combined  gases  and  salts,  of  which  carbonic  acid  is 
by  far  the  most  abundant,  constant,  and,  from  our  standpoint, 
important.  Still  smaller  quantities  of  ammoniacal  vapors  exist, 
and  in  volcanic  regions  there  have  been  detected  appreciable  but 
variable  quantities  of  sulphuric  and  hydrochloric  and  nitric  acids 
as  well.  With  rare  exceptions  these  last  exist  in  combination 
as  sulphates,  chlorides,  and  nitrates  and  with  the  exception  of 
the  last-named  need  little  consideration. 

(1)  Nitrogen,  Nitric  Acid,  and  Ammonia.  —  Nitrogen,  by  it- 
self, is  believed  to  be  wholly  inoperative  in  promoting  rock 
decomposition.  In  works  on  agricultural  chemistry,  much  has, 
however,  been  written  concerning  the  presence  in  the  atmos- 
phere of  the  compounds  of  nitrogen,  nitric  acid,  and  ammonia, 
and  it  will  be  well  to  devote  a  little  space  to  a  consideration  of 
the  facts  as  known,  and  their  possible  application  to  the  subject 
under  consideration. 

The  well-known  experiments  of  Cloez,  Boussingault,  De  Luca, 
Kletzinsky,  and  Way,  as  well  as  the  recent  ones  of  G.  H. 
Failyer,1  prove  conclusively  the  existence  of  ammonia  and  nitric 
acid  in  the  air,  from  whence  it  is  brought  to  the  surface  of  the 
earth  in  the  water  of  rainfalls. 

1  Ammonia  and  Nitric  Acid  in  Atmospheric  Waters,  2d  Ann.  Rep.  Kansas 
Experiment  Station,  1889. 


ACTION   OF   THE   ATMOSPHEHE 


17T 


In  nearly  every  case,  however,  the  percentage  of  ammonia,  as 
determined,  equalled  or  exceeded  the  amount  necessary  to  com- 
bine with  the  acid,  forming  thus  the  salt  ammonium  nitrate. 
Failyer's  experiments  in  Kansas,  carried  on  for  a  period  of  four 
years,  during  which  time  water  was  collected  from  266  rain- 
falls, showed  in  but  seven  instances  nitric  acid  equalling  or 
exceeding  the  ammonia.  In  all  other  cases  the  amount  is  less, 
with  the  possible  exception  of  the  reported  occurrence  (at 
Nismes,  in  1845)  of  a  fall  of  hail  sufficiently  acid  to  be  sour 
to  the  taste.  As  direct  promoters  of  rock  decomposition, 
neither  atmospheric  nitrogen  nor  free  nitric  acid  need,  then, 
serious  attention.  The  following  tables  are,  however,  of  inter- 
est, the  first  being  abridged  from  Johnson's  How  Crops  Feed, 
and  the  second  from  Professor  Failyer's  paper  above  quoted. 


AMOUNTS  or  RAIN  AND  OF  AMMONIA,  NITRIC  ACID,  AND  TOTAL  NITROGEN  THEREIN, 

COLLECTED  AT   ROTHAMSTEDD,   ENGLAND,   IN  THE   YEARS   1855-56,  CALCULATED 
PER    ACRE,   ACCORDING    TO    MESSRS.    LAWES,   GILBERT,  AND   WAY. 


Total   .     . 

Quantity  of  rain  in 
Imperial  gallons. 
1  gal.  =  10  Ib.  water 

Ammonia 
(in  pounds) 

Nitric  Acid 
(in  pounds) 

Total  Nitrogen 
(in  pounds) 

1855 

663.332 

1856 

616.051 

1855 

7.11 

1856 
9.53 

1855 

2.98 

1856 

2.80 

1855 
6.63 

1856 

8.31 

AMOUNTS  or  RAIN  AND  OF  AMMONIA,  NITRIC  ACID,  AND  NITROGEN  THEREIN,  COL- 
LECTED AT  MANHATTAN,  KANSAS,  1887-90,  ACCORDING  TO  G.  H.  FAILYER. 


Total  Nitrogen. 
Means  for  4 

Nitrogen  in 
ammonia. 
Means  for  3 

Nitrogen  in 
nitric  acid. 
Means  for  3 

years 

years 

Parts  per  million  of  water    .... 

0.522 

.388 

0.156 

Grammes  per  acre  .               ... 

1563.0 

1196.0 

480.0 

Pounds  per  acre      

3.44 

2.63 

1.06 

It  has  been  demonstrated,  however,  that  nitrogen  compounds 
and  nitrogenous  matter  in  the  soil  may  become  subject  to  nitri- 
fication through  the  action  of  bacteria,  whereby  ammonia, 
nitrous  or  nitric  acid,  carbon  dioxide,  and  water  are  formed, 


178       THE   PRINCIPLES   INVOLVED   IN   ROCK-WEATHERING 

though,  as  Wiley  says,  "  The  ammonia  and  nitrous  acid  may 
not  appear  in  the  soils,  as  the  nitric  organism  attacks  the  latter 
at  once  and  converts  it  into  nitric  acid."1  (See  further  under 
influence  of  plant  and  animal  life,  p.  203.) 

In  considering  the  possible  efficacy  of  these  compounds,  one 
must  not  lose  sight  of  the  fact  that  the  amount  of  nitrogen  in 
the  soils  is  as  a  rule  far  too  small  to  supply  the  demands  of 
growing  plants,  and  it  is  probable  that  a  very  large  proportion 
of  that  which  finds  its  way  there  is  quickly  taken  up  again  by 
these  organisms.  It  is  possible  that  other  salts  of  ammonium 
than  the  nitrate  may  be  locally  efficacious.  Thus  M.  Beyer, 
as  quoted  by  Van  Den  Broeck,2  has  shown  that  the  feldspars 
decompose  very  rapidly  under  the  influence  of  water  contain- 
ing ammonium  sulphate  or  even  sodium  chloride,  either  of 
which  substance  may  be  found  in  vegetable  soil.  Daubree, 
who  experimented  by  means  of  revolving  iron  cylinders  (see 
p.  197),  found,  however,  that  the  presence  of  sodium  chloride 
retarded  decomposition. 

(2)  Carbonic  Acid.  —  The  amount  of  carbonic  acid  in  the  air 
under  natural  conditions  is  not  a  widely  variable  quantity,  ex- 
cepting near  volcanoes  and  the  immediate  vicinity  of  gaseous 
springs.  In  the  vicinity  of  large  cities  and  manufactories 
consuming  great  quantities  of  coal,  the  amount  is  naturally 
increased.  Although  carbonic  acid  is  the  most  abundant  gas 
given  off  by  decomposing  vegetable  matter,  it  has  apparently 
been  definitely  ascertained  that  the  amount  of  this  gas  in  regions 
of  abundant  vegetation  is  no  greater  than  elsewhere.  This  has 
been  accounted  for  on  the  assumption  that,  as  fast  as  liberated,  it 
is  taken  up  by  growing  organisms  or  carried  by  rains  into  the  soil.3 

1  Wiley,  Principles  and  Practice  of  Agricultural  Analysis. 

2  Mem.  sur  les  phenomenes  d' Alteration  des  Depots  Superficial,  p.  16. 

3  The  researches  of  Boussingault  and  Lewey   (Mem.  de  Chemie  Agricole, 
etc.),  as  quoted  by  Johnson  (How  Crops  Feed,  p.  139),  showed  the  following 
proportions  existing  between  the  C02  of  the  air  of  the  atmosphere  and  of  various 
soils :  — 

C02  IN  10,000  PARTS 
BY  WEIGHT 

Ordinary  atmosphere 6  parts 

Air  from  sandy  subsoil  of  forest 38  parts 

Air  from  loamy  subsoil  of  forest 124  parts 

Air  from  surface  soil  of  forest 130  parts 

Air  from  surface  soil  of  vineyard 146  parts 

Air  from  pasture  soil 270  parts 

Air  from  soil  rich  in  humus 543  parts 


ACTION   OF   THE   ATMOSPHERE  179 

Twenty-one  tests  of  the  air  in  various  parts  of  Boston,  during 
the  spring,  1870,  showed  the  presence  of  385  parts  of  carbonic 
acid  in  1,000,000.  Eleven  tests  of  the  winter  air  in  Cambridge 
yielded  337  parts  in  1,000,000.!  Dr.  J.  H.  Kidder  found  the 
out-door  air  of  Washington  to  contain  387  to  448  parts  in 
1,000,000,  while  Dr.  Angus  Smith,  after  an  elaborate  series  of 
experiments,  reported  the  atmosphere  of  Manchester  (Eng- 
land) as  containing  442  parts  in  1,000,000. 2 

These  amounts  are  considerably  in  excess  of  those  reported 
by  Miintz  and  Aubin,3  who  give  the  following  figures  relative 
to  the  proportional  amounts  in  10,000  by  volume,  as  determined 
at  the  various  widely  separated  stations.  The  amount,  it  will 
be  perceived,  is  slightly  greater  during  the  night  than  during 
the  day. 

DAY  NIGHT 

Hayti 2.704  2.920 

Florida 2.897  2.947 

Martinique 2.735  2.850 

Mexico 2.665  2.860 

Santa  Cruz,  Patagonia 2.664  2.670 

Chubut,  Patagonia 2.790  3.120 

Chili 2.665  2.820 

The  general  mean  is  then  2.78  parts  in  10,000,  that  for  the 
night  alone  being  2.82.  For  the  north  of  France  the  mean  is 
given  as  2.962,  for  the  plain  of  Vincennes  2.84,  and  for  the 
summit  of  the  Pic  du  Midi  2.86. 

Fischer,  as  quoted  by  Branner,4  has  shown  that  in  rain  and 
snow  water  the  amount  of  carbonic  acid  varies  between  0.22% 
and  0.45  %  by  volume  of  water.  Assuming  that  the  mean  of 
these  figures  fairly  represents  the  general  average,  it  is  easy, 
knowing  the  rainfall  of  any  region,  to  calculate  the  amount  of 
the  gas  thus  annually  brought  to  the  surface.  Professor  Bran- 
ner has  thus  calculated  that  from  3.21  to  11.80  millimetres  of 
carbonic  acid  (CO2)  are  annually  brought  to  the  surface  in  cer- 
tain parts  of  Brazil.  The  same  method  of  calculation  applied 
to  the  various  parts  of  the  United  States,  would  give  us  for  the 
Atlantic  coast  states  3.75  mm.;  for  the  upper  Mississippi  val- 
ley, 2.50  mm.;  for  the  lower  Mississippi  valley,  4.50  mm.  ;  and 

1  2d  Ann.  Rep.  Mass.  State  Board  of  Health,  1871. 

2  Air  and  Rain,  p.  52. 

3  Comptes  Rendus,  Vol.  XCIII,  1881,  p.  797  ;   also  XCVI,  1883,  pp.  1793-97. 

4  Op.  cit. 


180      THE   PRINCIPLES   INVOLVED   IN   ROCK- WEATHERING 

for  the  northern  Pacific  states,  6.25  mm.  As  it  is  mainly  when 
this  carbonic  acid  is  thus  brought  to  the  surface  by  the  rain 
and  snows  that  its  effects  become  of  direct  significance  in  our 
present  work,  the  matter  may  be  dropped  here,  to  be  taken  up 
again  when  considering  the  chemical  action  of  water. 

(3)  Oxygen.  —  Under  ordinary  conditions  oxygen  is  the  most 
active  principle  in  atmospheric  air,  and  it  is  to  this  agent  that 
is  due  the  process  of  oxidation  which  almost  invariably  char- 
acterizes the  decomposition  of  silicates  and  other  minerals  con- 
taining iron  in  the  protoxide  state.     Such  oxidation  is,  however, 
almost  inactive  unless  aided  by  moisture,  and  a  further  discus- 
sion of  the  subject  may  well  be  deferred,  to  be  taken  up  again 
when  discussing  the  action  of  water. 

(4)  Heat  and  Cold.  —  The  ordinarily  feeble  action  of  the  air 
is  greatly  augmented  through  natural  temperature  variations. 
That  heat  expands  and  cold  contracts  is  a  fact  too  well  known 
to  need   elaboration.     That,  however,  the  constant  expansion 
and  contraction  due  to  diurnal  temperature  variations  may  be 
productive  of  weakness  and  ultimate  disintegration  in  so  inert 
a  body  as  stone,  seems  not  so  generally  understood,  or  is,  at 
least,  less  well  appreciated,  and  hence  a  little  space  is  devoted 
to  the  subject  here.     Rocks,  it  must  be  remembered,  as  the 
writer  has  noted  elsewhere,1  are  complex  mineral  aggregates  of 
low  conducting  power,  each  individual  constituent  of  which 
possesses  its  own  ratio  of  expansion,  or  contraction,  as  the  case 
may  be.     In  crystalline  rocks  these  various  constituents  are 
practically  in  contact.     In  clastic  rocks  they  are,  on  the  other 
hand,  frequently  separated  from  one  another  by  the  interposi- 
tion of  a  thin  layer  of  calcareous,  ferruginous,  or  siliceous  matter 
which  serves  as  a  cement.     As  temperatures  rise,  each  and  every 
constituent  expands  and  crowds  with  almost   resistless   force 
against  its  neighbor  ;  as  temperatures  fall,  a  corresponding  con- 
traction takes  place.     Since  in  but  few  regions  are  surface  tem- 
peratures constant  for  any  great  period  of  time,  it  will  be  readily 
perceived  that  almost  the  world  over  there  must  be  continuous 
movement  within  the  superficial  portions  of  the  mass  of  a  rock. 

The  actual  amount  of  expansion  and  contraction  of  stone 
under  ordinary  temperatures  has  been  a  matter  of  experiment. 
W.  H.  Bartlett 2  has  shown  that  the  average  rate  of  expansion 

1  Stones  for  Building  and  Decoration,  Wiley  &  Sons,  New  York. 

2  Am.  Jour,  of  Science,  Vol.  XXII,  1832,  p.  136. 


ACTION   OF  THE   ATMOSPHERE  181 

for  granite  amounts  to  .000004825  inch  per  foot  for  each  de- 
gree Fahrenheit ;  for  marble  .000005668  inch,  and  for  sandstone 
.000009532  inch.  Adie,  in  a  series  of  similar  experiments, 
found  the  rate  of  expansion  for  granite  to  be  .00000438  inch, 
and  for  white  marble  .00000613  inch.1  Slight  as  these  move- 
ments may  seem,  they  are  sufficient  to  in  time  produce  a  decided 
weakening  and  afford  a  starting-point  for  other  physical  and 
chemical  agencies,  such  as  are  ever  lying  in  wait  for  an  oppor- 
tunity to  get  in  their  work.  The  writer  well  remembers  the 
peculiar  impressions  produced  during  one  of  his  earlier  trips 
into  the  comparatively  arid  regions  of  Montana,  at  finding,  at  a 
certain  place,  the  slopes  and  valley  bottoms  strewn  with  small, 
beautifully  fresh,  concave  and  convex  chips  of  a  dense,  coal- 
black,  andesitic  rock  that  occupied  the  crest  of  one  of  the  higher 
hills.  So  fresh  were  the  fractures,  so  free  were  they  from  oxi- 
dation or  other  signs  of  decomposition,  it  was  at  first  felt  that 
they  must  be  of  human  origin,  that  they  were  chips  flaked  off 
by  aboriginal  workmen  in  making  stone  implements,  and  some 
time  was  wasted  in  seeking  for  the  more  complete  results  of 
their  handiwork.  It,  however,  did  not  take  long  to  convince 
him  that  the  flakes  were  far  too  abundant  and  too  widely  spread 
to  have  originated  in  any  such  way,  while  the  finding,  on  the 
top  of  the  hill,  of  the  coal-black  rock,  broken  into  larger  colum- 
nar blocks,  each  with  its  angles  rendered  more  obtuse  or  even 
fluted  by  the  springing  off  of  just  such  flakes,  —  this,  coupled 
with  the  knowledge  that  during  the  day,  exposed  under  a 
cloudless  sky,  the  rocks  became  so  highly  heated  as  to  be  un- 
comfortable to  the  touch,  whilst  at  night  the  temperature  sank 
nearly  to  the  freezing-point,  sufficed  to  teach,  as  it  must  have 
taught  the  most  obtuse,  that  the  ordinary  daily  temperature 
variations  were  amply  sufficient  to  account  for  the  phenom- 
enon. 

Shaler  states  2  that  rock  surfaces  in  the  eastern  United  States 
may  be  subjected  to  temperatures  varying  from  150°  F.  at 
midday  in  summer  to  0°  and  below  in  winter.  This  change  of 
150°  in  a  sheet  of  granite  100  feet  in  diameter  would  produce  a 
lateral  expansion  of  about  one  inch  of  surface.  That  this  ex- 
pansion must  tend  to  lessen  the  cohesion  and  tear  the  upper 
from  the  deeper  lying  layers,  is  self-evident.  As  exemplifying 

1  Trans.  Royal  Soc.  of  Edinburgh,  Vol.  XIII,  p.  366. 

2  Proc.  Boston  Soc.  of  Nat.  History,  XII,  1809,  p.  292. 


182      THE  PRINCIPLES   INVOLVED   IN  ROCK-WEATHERING 

this,  Professor  Shaler  states  that  there  are  on  Cape  Ann  (Massa- 
chusetts) hundreds  of  acres  of  bare  rock  surface  completely 
covered  with  blocks  of  stone,  which  have  been  separated  from 
the  mass  beneath  by  just  this  process.1 

The  size  of  such  flakes  may  vary  from  those  of  microscopic 
proportions  to  masses  of  several  tons'  weight.  The  higher 
slopes  of  Lone  Mountain,  east  of  the  Madison,  in  Montana, 
are  covered  above  timber  line  with  thousands  upon  thousands 
of  these  loose  flakes  of  all  sizes  up  to  ten  or  more  feet  in  diam- 
eter. Such,  here,  as  in  general,  are  characterized  by  a  roughly 
lenticular  outline  in  cross-section,  possessing  a  large  superficial 
area  in  proportion  to  their  thickness,  and  are  further  distin- 
guished from  boulders  of  decomposition  by  the  entire  freshness 
of  their  materials  even  to  the  very  surface.  In  close-grained, 
black  andesitic  and  basaltic  rocks  the  chip  or  flake  not  infre- 
quently shows  a  beautiful  concave  and  convex  form  and  is 
greatly  elongated  in  proportion  to  its  breadth,  resembling  the 
long  and  slender  chips  of  obsidian  or  flint  found  on  the  sites 
of  aboriginal  workshops.  The  surface  left  by  the  springing  off 
of  these  flakes  is  of  course  fluted  as  though  the  work  were 
done  with  a  carpenter's  gouge. 

It  is  natural  that  this  form  of  disintegration  should  be  most 
pronounced  in  massive,  close-grained  rocks.  In  regions  of  great 
extremes  of  daily  temperature  the  rupturing  of  these  masses 
from  the  parent  ledge  is  frequently  attended  by  gun-like  reports 
sufficiently  loud  to  be  heard  at  a  considerable  distance.  H.  von 
Streeruwitz  states2  that  the  rocks  of  the  Trans  Pecos  (Texas) 
region  undergo  a  very  rapid  disintegration  from  diurnal  tem- 
perature variations,  which  here  amount  to  from  60°  to  75°  F. 
He  says:  "I  frequently  observed  in  summer,  as  well  as  in  winter 
time,  on  the  heights  of  the  Quitman  Mountains  a  peculiar  crack- 
ling noise  and  occasionally  loud  reports,  .  .  .  and  careful 

1  The  rifting  action  of  heat  upon  granitic  masses  is  said  to  have  been  made  a 
matter  of  quarry  utility  in  India.    It  is  stated  (Nature,  January  17,  1895)  that  a 
wood  fire  built  upon  the  surface  of  the  granite  ledge  and  pushed  slowly  forward 
causes  the  stone  to  rift  out  in  sheets  six  inches  or  so  in  thickness,  and  of  almost 
any  desired  superficial  area.     Slabs  60  x  40  feet  in  area,  varying  not  more  than 
half  an  inch  from  a  uniform  thickness  throughout,  have  been  thus  obtained.    In 
one  instance  mentioned,  the  surface  passed  over  by  the  line  of  fire  was  460  feet, 
setting  free  an  area  of  stone  of  740  square  feet  of  an  average  thickness  of  five 
inches.     This  stone  is  undoubtedly  one  of  remarkably  easy  rift,  but  the  case 
will,  nevertheless,  serve  our  present  purposes  of  illustration. 

2  4th  Ann.  Rep.  Geol.  Survey  of  Texas,  1892,  p.  144. 


ACTION  OF  THE   ATMOSPHERE  183 

research  revealed  the  fact  that  the  crackling  was  caused  by  the 
gradual  disintegration  and  separation  of  scales  from  the  surface 
of  the  rock,  and  the  loud  reports  by  crackling  and  splitting  of 
huge  boulders."  The  scales  thus  split  off,  he  says,  vary  in 
thickness  from  one-half  to  four  inches,  and  their  superficial 
area  from  a  few  square  inches  to  many  feet.  This  form  of 
disintegration  is  necessarily  confined  to  slopes  unprotected  by 
vegetation,  and  is  the  more  pronounced  the  greater  the  diurnal 
variations. 

In  Arabia  Petrea,  according  to  Marsh,1  "  when  a  wind  pow- 
erful enough  to  scour  down  below  the  ordinary  surface  of  the 
desert  and  lay  bare  a  fresh  bed  of  stones  is  followed  by  a  sudden 
burst  of  sunshine,  the  dark  agate  pebbles  are  often  cracked  and 
broken  by  the  heat."  According  to  Livingstone,  the  rock  tem- 
peratures in  certain  parts  of  Africa,  on  the  immediate  surface, 
rise  during  the  day  as  high  as  137°  F.  and  at  night  fall  so  rap- 
idly as  to  throw  off  by  their  contraction  sharp,  angular  masses 
in  sizes  up  to  200  pounds'  weight.  Stanley,  in  his  reports, 
is  inclined  to  lay  considerable  stress  on  the  effects  of  cold  rains 
upon  the  heated  rock  surfaces,  though  it  is  doubtful  if  this. is 
as  powerful  an  agent  as  his  descriptions  would  give  us  to  under- 
stand. (See  further  under  action  of  water.)  Throughout  the 
desert  regions  of  lower  California,  as  observed  by  the  writer, 
the  granitic  and  basic  eruptive  rocks  subject  to  very  little 
rainfall,  and  hence  almost  completely  bare  of  vegetation,  under 
the  blistering  heat  of  the  desert  sun  have  weathered  down  into 
dome-shaped  masses,  their  debris  in  the  form  of  angular  bits  of 
gravel  being  strewn  over  the  plain.  Particles  of  this  gravel, 
when  compared  with  those  which*  are  a  product  of  chemical 
agencies,  are  found  to  differ  in  that  each,  however  friable,  is  a 
complex  molecule  of  quartz,  feldspar  and  mica  or  other  mineral 
that  may  have  composed  the  rock  from  which  it  was  derived. 
Aside  from  a  whitening  of  the  feldspathic  constituent,  due  to 
the  reflection  of  the  light  from  its  parted  cleavage  planes, 
scarcely  any  change  has  taken  place,  and  indeed  it  more  resem- 
bles the  finely  comminuted  material  from  a  rock-crusher  than 
a  product  of  natural  agencies. 

Owing,  however,  to  the  low  conducting  power  of  rocks,  dis- 
integration from  this  cause  alone  can  go  on  to  any  extent  only 
at  the  immediate  surface,  and  on  flat  and  level  plains,  where 
1  The  Earth  .as  modified  by  Human  Action,  p.  552. 


184      THE   PRINCIPLES  INVOLVED   IN  KOCK-WEATHERING 


the  debris  is  allowed  to  accumulate,  must  in  time  completely 
cease.1  It  is  only  on  hillsides  and  slopes,  or  where  by  the 
erosive  action  of  running  water,  or  by  wind,  the  debris  is 
removed  as  fast  as  formed,  that  such  can  have  any  geological 
significance,  although  the  rate  of  such  disintegration  is  suffi- 
ciently rapid  in  exposed  places  to  be  of  serious  consequence  in 
stone  used  for  architectural  application.  (See  further  on  p.  198, 
Action  of  Ice.) 

(5)  Wind.  —  But  it  is  to  the  action  of  the  air  when  in  motion 
—  to  the  wind  —  that  is  due  a  very  considerable  part  of  atmos- 
pheric work.  Particles  of  sand  drifting  along  before  the  wind 
become  themselves  agents  of  abrasion,  filing  away  on  every  hard 
object  with  which  they  come  in  contact.  As  a  matter  of  course, 
this  phenomenon  is  most  strikingly  active  in  the  arid  regions, 
though  the  results,  when  looked  for,  are  by  no  means  wanting 
in  the  humid  east.  It  is  thought  by  Professor  Egleston  that 
many  of  the  tombstones  in  the  older  churchyards  of  New  York 
City  have  become  illegible  by  the  wearing  action  of  the  dust 
and  sand  blown  against  them  from  the  street.  There  is  among 

1  Observations  on  soil  temperatures  made  at  the  Orono,  Maine,  Experimental 
Station  showed  that  the  mean  daily  range  of  temperatures  from  April  to  Octo- 
ber, at  a  depth  below  the  surface  of  1  inch,  was  5.62° ;  at  a  depth  of  3  inches, 
5.26°  ;  at  6  inches,  1.9° ;  and  at  9  inches,  1.18°  ;  and  at  12  inches  very  slight.  At 
the  depth  of  1  inch  the  temperature  was  lower  than  that  of  the  air  by  2.4° ;  at 
3  inches  by  2.11° ;  at  6  inches  by  3.16° ;  at  9  inches  by  3.94° ;  at  12  inches  by 
4.18°  ;  at  24  inches  by  5.78°  ;  and  at  36  inches  by  7.10°. 

The  following  table,  compiled  by  Forbes  (Trans.  Royal  Society  of  Edinburgh, 
Vol.  XVI,  1849),  from  observations  made  near  Edinburgh,  Scotland,  during 
1841-42,  shows  the  range  of  earth  temperatures  at  varying  depths  in  soil,  sand- 
stone, and  trap  rock. 


TRAP  EOCK 

SAND  OF  GARDEN 

CRAIGLEITII  SANDSTONE 

DEPTH 

Max. 

Min. 

Range 

Max. 

Min. 

Range 

Max. 

Min. 

Range 

3  feet  .    .    . 

52.85° 

38.88° 

13.97° 

54.50° 

37.85° 

17.65° 

53.15° 

38.25° 

14.90° 

6  feet  .    .    . 

51.07 

40.78 

10.29 

52.95 

39.55 

13.40 

51.90 

38.95 

12.95 

12  feet     .    . 

49.00 

44.20 

4.80 

50.40 

43.50 

6.90 

50.30 

41.60 

8.70 

24  feet     .    . 

47.50 

46.12 

1.38 

48.10 

46.10 

2.00 

48.25 

44.35 

3.90 

It  has  been  shown  that  the  thermal  conductivity  of  rocks  varies  in  direction 
according  to  their  structure,  being  greatest  in  the  direction  of  their  schistosity, 
where  such  exists.  In  massive,  homogeneous  rocks  the  conductivity  is  the  same 
in  all  directions.  In  finely  fissile  rocks,  on  the  other  hand,  it  may  be  four  times 
as  great  in  the  direction  of  their  fissility  as  at  right  angles  thereto. 


ACTION   OF  THE    ATMOSPHERE  185 

the  heterogeneous  collections  of  the  National  Museum  at  Wash- 
ington a  large  sheet  of  plate  glass,  once  a  window  in  a  light- 
house on  Cape  Cod.  During  a  severe  storm,  of  not  above  forty- 
eight  hours'  duration,  this  became  on  its  exposed  surface  so 
ground  from  the  impact  of  grains  of  sand  blown  against  it  as  to 
be  no  longer  transparent,  and  to  necessitate  its  removal.  Win- 
dow panes  in  the  dwelling-houses  of  the  vicinity  are,  it  is  stated, 
not  infrequently  drilled  quite  through  by  the  same  means. 

Apply  now  this  agency  to  a  geological  field  in  a  dry  region. 
The  wind,  sweeping  across  a  country  bare  of  verdure  and 
parched  by  drought,  catches  up  the  loose  particles  of  dust  and 
sand  and  drives  them  violently  into  the  air  in  clouds,  or  sweeps 
them  along  more  quietly  close  to  the  surface,  where  they  are 
at  first  scarce  noticeable.  The  impact  of  a  single  one  of  these 
moving  grains  on  any  object  with  which  it  may  come  in  con- 
tact is  far  too  small  to  be  appreciable ;  but  the  impact  of 
millions,  acting  through  days,  weeks,  and  years,  produces  re- 
sults not  merely  noticeable,  but  strikingly  conspicuous.  We 
have  here,  in  fact,  a  natural  sand  blast,  an  illustration  on  a 
grand  scale  of  a  principle  in  common  use  in  glass-cutting,  and 
to  a  small  extent  in  stone-cutting  also.  Constantly  filing  away 
on  every  object  with  which  they  come  in  contact,  the  grains 
go  sweeping  on,  undermining  cliffs,  scouring  down  mountain 
passes,  wearing  away  the  loose  boulders,  and  smoothing  out  all 
inequalities.  Naturally  the  abrading  action  on  exposed  blocks 
of  stone  is  most  rapid  near  the  ground,  as  here  the  flying  sand 
grains  are  thickest.  First  the  sharp  angles  and  corners  are 
worn  away,  and  the  masses  gradually  become  pear-shaped, 
standing  on  their  smaller  ends.  Finally  the  base  becomes 
too  small  for  support,  the  stone  topples  over,  and  the  process 
begins  anew  without  a  moment's  intercession,  and  continues 
until  the  entire  mass  disappears, — becomes  itself  converted 
into  loose  sand  drifted  by  the  wind  and  an  agent  for  destruc- 
tion. Professor  W.  P.  Blake  was  the  first,  I  believe,  to  call  pub- 
lic attention  to  this  phenomenon,  having  observed  it  while  in  the 
Pass  of  San  Bernardino  (California)  in  1853.  G.  K.  Gilbert 
has  also  published  some  interesting  facts  as  noted  by  himself 
while  geologist  of  the  Wheeler  Expedition  west  of  the  100th 
meridian,  in  1878.1  In  acting  on  the  hard  rocks,  the  sand  cuts 

1  It  should  be  noted  that  the  "sand-blast  carving"  described  by  Gilbert  in 
this  report  is  not  due  wholly  to  the  action  of  wind-blown  sand.    The  rock  is  fine 


186       THE  PRINCIPLES   INVOLVED   IN   ROCK-WEATHERING 

so  slowly  as  at  times  to  produce  only  grooved  or  fantastically 
carved  surfaces,  often  with  a  very  high,  polish.  The  geologists 
of  the  40th  Parallel  Survey  in  1878  described  like  interesting 
phenomena  as  observed  on  the  western  faces  of  conglomerate 
boulders  exposed  to  the  sand  blast  of  the  desert  regions  'of  Ne- 
vada. The  surface  of  the  otherwise  light-colored  rock  was 
found  to  have  assumed  a  dark  lead-gray  hue  and  a  polish  equal 
to  that  of  glass,  while  the  sand  had  drilled  irregular  holes  and 
grooves,  often  three-fourths  of  an  inch  deep  and  not  more  than 
an  eighth  of  an  inch  in  diameter,  through  pebbles  and  matrix 
alike.  Professor  W.  M.  Davis,1  G.  H.  Stone,2  and  J.  B.  Wood- 
ward 3  have  described  pebbles  occurring  in  the  glacial  deposits 
of  Cape  Cod  and  of  Maine,  carved  and  facetted  by  the  same 
agencies. 

2.     CHEMICAL  ACTION  OF  WATER 

Pure  water,  although  an  almost  universal  solvent,  neverthe- 
less acts  with  such  slowness  upon  the  ordinary  materials  of  the 
earth's  crust,  that  its  results  are  scarcely  appreciable  to  the 
ordinary  observer.  But  it  by  no  means  follows  that  its  effects 
are  not  worthy  of  our  consideration  here.  This  is  particularly 
true  when  we  reflect  that  the  results  being  discussed  are  not 
merely  those  of  days  and  weeks,  but  of  years  even  when  counted 
by  the  tens  of  thousands  and  millions.  Moreover,  absolutely 
pure  water,  as  a  constituent  of  our  sphere,  presumably  does  not 
exist.  We  have  to  consider  its  action  as  well  when  contami- 
nated with  sundry  salts  and  acids  which  it  almost  universally 
holds,  having  taken  them  up  in  passing  through  the  atmosphere, 
and  in  filtering  through  the  overlying  layer  of  organic  matter 
and  decomposition  products  which  cover  so  large  a  portion  of 
the  surface  of  the  land.  It  is  when  thus  contaminated  that  are 
manifested  the  wonderful  solvent  and  other  chemical  reactions 
which  have  been  instrumental  in  promoting  rock  destruction, 
and  it  is  here,  then,  that  will  be  considered  the  complex  chemical 

calcareous  shale.  Through  the  solvent  action  of  meteoric  water  the  calcareous 
cement  is  removed,  the  fine,  argillaceous  interstitial  material  mechanically 
eroded,  while  the  more  resisting  granules  of  quartz  sand  stand  in  relief,  giving 
rise  to  elevated  points  and  ridges. 

1  Proc.  Boston  Soc.  of  Natural  History,  Vol.  XXVI,  1893,  p.  166. 

2  Am.  Jour.  Science,  Vol.  XXXI,  1886,  p.  133. 

3  Ibid.,  Jan.,  1894,  p.  63. 


CHEMICAL   ACTION   OF    WATER  187 

processes  commonly  grouped  under  the  head  of  oxidation,  deoxi- 
dation,  hydration,  and  solution. 

(1)  Oxidation.  —  Oxidation  is  perceptibly  manifested  only  in 
rocks  carrying  iron  either  as  sulphide,  protoxide  carbonate,  or 
silicate.     The  sulphides,  in  presence  of  water  and  when   not 
fully  protected  from  atmospheric  influences,  readily  succumb, 
producing  sulphates  which,  being  soluble,  are  removed  in  solu- 
tion, or  hydrated  oxides,  sulphuretted  hydrogen,  and  perhaps 
free  sulphur,  as  already  noted  (p.  29).     Such  an  oxidation  is 
attended  by  an  increase  in  bulk,  so  that  if  nothing  escapes  by 
solution,  there  may  be  brought  to  bear  a  physical  agency  to  aid 
in  disintegration.     Weathered  rocks,  containing  iron  sulphides, 
may  not  infrequently  be  found  with  cubical  cavities  quite  empty 
or  partially  filled  with  the  brownish,  yellow,  or  red  product  of 
its  oxidation  in  a  more  or  less  powdery  condition.     Pyrites, 
though  a  wide-spread  constituent,  is,  nevertheless,  a  less  con- 
spicuous agent  in  promoting  rock  decomposition  than  the  pro- 
toxide carbonates  and  silicates.     In  these  the  iron  passes  also 
over  to  the  hydrated  sesquioxide  state,  as  is  indicated  by  the 
general  discoloration,  whereby  the  rock  becomes  first  streaked 
and  stained,  and  finally  uniformly  ochreous.     The  more  com- 
mon minerals  thus  attacked  are  the  ferruginous  carbonates  of 
lime  and  magnesia,  and  silicates  of  the  mica,  amphibole,  and 
pyroxene  groups.     As  the  oxidation  progresses,  the  minerals 
become  gradually  decomposed  and  fall  away  into  unrecogniz- 
able forms.     The  red  and  yellow  colors  of  soils  are  due  invari- 
ably to  the  iron  oxides  contained  by  them.     In  many  cases,  the 
mineral  magnetite,  a  mixture  of  proto-  and  sesqui-oxides,  under- 
goes further  oxidation  and  also  loses  its  individuality. 

(2)  Deoxidation  is  a  less  common  feature  than  oxidation. 
Water,  carrying   small  quantities  of   organic   acids,  may  take 
away   a   portion   of   the   combined   oxygen  of   a   sesquioxide, 
converting  it  once  more  into  the  protoxide  state.      The  local 
bleaching  of  certain  ferruginous  sands  and  sandstones  is  due 
to   this  action  and  a  partial  removal  of  the  ferriferous  salt 
in  solution.     Through  a  similar  process  of  deoxidation,  ferrous 
sulphates  may  be  converted  into  sulphides,  a  process  which 
undoubtedly  takes  place  in  marine  muds  protected  from  atmos- 
pheric action. 

(3)  Hydration  —  the  assumption  of  water  —  more  commonly 
accompanies  oxidation,  and,  indeed,  is  an  almost  constant  accom- 


188      THE   PRINCIPLES   INVOLVED   IN   ROCK-WEATHERING 

paninient  of  rock  decomposition,  as  may  be  observed  in  com- 
paring the  total  percentages  of  water  in  fresh  and  decomposed 
minerals  and  rocks,  as  given  in  the  analyses. 

This  assumption,  provided  it  be  not  accompanied  by  a  loss  of 
constituents,  either  by  solution  or  erosion,  must  be  attended  by 
an  increase  in  bulk,  such  as  may  be  quite  appreciable.  The 
Comte  de  la  Hure,  as  quoted  by  Branner,1  has  expressed  the 
opinion  that  some  of  the  hills  of  Brazil  have  actually  increased 
in  height  through  this  means.  The  present  writer  has  calcu- 
lated that  the  transition  of  a  granitic  rock  into  arable  soil,  pro- 
vided the  same  took  place  without  loss  of  material,  must  be 
attended  by  an  increase  in  bulk  amounting  to  88  % . 

Hydration  as  a  factor  in  rock  disintegration  is,  in  the  writer's 
opinion,  of  more  importance  than  is  ordinarily  supposed.  Granitic 
rocks  in  the  District  of  Columbia  have  been  shown  2  to  have  be- 
come disintegrated  for  a  depth  of  many  feet  with  loss  of  but 
comparatively  small  quantities  of  their  chemical  constituents 
and  with  apparently  but  little  change  in  their  form  of  combina- 
tion. Aside  from  its  state  of  disintegration,  the  newly  formed 
soil  differs  from  the  massive  rock,  mainly  in  that  a  part  of  its 
feldspathic  and  other  silicate  constituents  have  undergone  a  cer- 
tain amount  of  hydration.  Natural  joint  blocks  of  the  rock 
brought  up  from  shafts  were,  on  casual  inspection,  sound  and 
fresh.  It  was  noted,  however,  that  on  exposure  to  the  atmos- 
phere such  shortly  fell  away  to  the  condition  of  sand.  Closer 
inspection  revealed  the  fact  that  the  blocks  when  brought  to  the 
surface  were  in  a  hydrated  condition,  giving  forth  only  a  dull, 
instead  of  clear,  ringing  sound,  when  struck  with  a  hammer,  and 
showing  a  lustreless  fracture,  though  otherwise  unchanged. 
That  such  had  not  previously  fallen  away  to  the  condition  of  sand 
was  evidently  due  to  the  vice-like  grasp  of  the  surrounding  rock 
masses.  These  observations  seem  to  have  since  received  confir- 
mation from  Professor  Derby,3  who  states  that  the  sedimentary 
rocks  of  Sao  Paulo,  Brazil,  as  seen  in  the  deep  railway  cuttings, 
"  are  almost  invariably  soft  even  when  they  show  no  signs  of 
decay,  and  go  to  pieces  by  a  kind  of  slaking  process  when 
broken  up  and  exposed  to  the  air,  though  they  may  have 
required  blasting  in  the  original  opening  of  the  cuttings." 

1  Op.  cit.,  p.  284. 

2  Bull.  Geol.  Soc.  of  America,  Vol.  VI,  p.  321. 

8  Decomposition  of  Rocks  in  Brazil,  Jour,  of  Geol.,  Vol.  IV,  1896,  p.  205. 


CHEMICAL   ACTION   OF   WATER  189 

Professor  W.  O.  Crosby l  gives  it  as  his  opinion  that  the  dis- 
integration of  the  Pike's  Peak  (Colorado)  granite  is  due  mainly 
to  hydration,  the  mica  particularly  being  affected. 

Professor  Alexander  Johnstone  showed 2  by  experimentation 
that  normal  muscovites,  when  submitted  to  the  action  of  pure 
and  carbonated  waters  for  the  space  of  a  year,  underwent  very 
little  change  other  than  hydration,  and  a  diminution  in  lustre, 
hardness,  and  elasticity.  They  appeared,  in  fact,  to  be  converted 
merely  into  hydromuscovites,  the  hydration  in  pure  water  hav- 
ing gone  on  nearly  as  rapidly  as  in  that  which  was  carbonated. 
Biotite,  when  similarly  treated,  showed  a  slight  discoloration 
or  bleaching  on  the  edges,  accompanied  also  by  hydration,  and, 
in  the  case  of  that  in  carbonated  water,  a  distinct  loss  of  iron 
and  magnesia  through  solution.  Lepidolite,  voigtite,  vermicu- 
lite,  and  pyrosclerite  were  similarly  acted  upon,  the  iron  and 
magnesia  being  removed  in  the  form  of  carbonates.  The  fact 
was  noted  "  that  whenever  anhydrous  micas,  or  lower  hydrated 
micas,  become  hydrated,  they  always  at  the  same  time  increase 
in  bulk."  This  fact  he  regarded  as  accounting  for  the  rapid 
weathering  of  micaceous  sandstones. 

(4)  Solution.  —  The  solvent  action  of  water  is  perhaps  the 
most  important  of  its  immediate  effects,  though  there  are  many 
incidental  chemical  changes  set  in  operation  which,  in  the  end, 
are  of  equal  or  even  greater  significance.  It  is  the  solvent 
action  only  that  concerns  us  here. 

Rain  and  nearly  all  superficial  waters  contain  small  quantities 
of  carbonic,  humic,  ulmic,  crenic,  and  apocrenic  acids,  which 
greatly  increase  their  solvent  capacities.  The  last-named  forms 
are  complex,  unstable,  and  little  understood  products  of  plant 
decomposition,3  and  might  logically  be  considered  under  effects 

1  Personal  Memoranda  to  the  Writer. 

2  Quar.  Jour.  Geol.  Soc.  of  London,  Vol.  XLV,  1889. 

3  The  following  are  the  chemical  formulas  of  these  acids,  as  commonly 
given:  — 

ULMIN  AND  ULMIC  ACID 

Carbon* C7.1°/U 

Hydrogen 4.2      I  Corresponding  to  C4oH28Oi2  +  H20 

Oxygen 8.7     J 

HUMIN  AND  HUMIC  ACID 

Carbon C4.4%\ 

Hydrogen 4.3      L  Corresponding  to  C2iH24Oi2 -f  3  H2O 

Oxygen 31.3     J 


190       THE   PRINCIPLES   INVOLVEP   IN  ROCK-WEATHERING 

of  plant  and  animal  life,  but  that  they  act  only  in  presence  of 
moisture. 

"  There  is  reason  to  believe  that  in  the  decomposition  effected 
by  meteoric  waters  and  usually  attributed  mainly  to  carbonic 
acid,  the  initial  stages  of  attack  are  due  to  the  powerful  solvent 
capacities  of  the  humus  acids.  Owing,  however,  to  the  facility 
with  which  these  acids  pass  into  higher  stages  of  oxidation,  it  is 
chiefly  as  carbonates  that  the  results  of  their  action  are  carried 
down  into  deeper  parts  of  the  crust  or  brought  up  to  the  sur- 
face. Although  CO2  is  ho  doubt  the  final  condition  into  which 
these  unstable  organic  acids  pass,  yet  during  their  existence 
they  attack  not  merely  alkalies  and  alkaline  earth,  but  even 
dissolve  silica."1  P.  Thernard  found  that  the  solvent  power 
of  these  acids  was  largely  controlled  by  the  amount  of  nitrogen 
they  contained.2 

CRENIC  ACID 
Carbon  .    44.0 


Oxygen 46.6 


APOCRENIC  ACID 

Carbon 34.4  %-, 

Hydrogen 3.5     I 

Nitrogen 3.0     f  Corresponding  to  CM 

Oxygen 39.1     J 

Berthelot  and  Andre  (Comptes  Rendus  Academie  de  Paris,  114,  1892,  pp.  41- 
43)  have  shown  that  the  brown  substance  of  humus  and  analogous  compounds 
undergo  direct  oxidation  under  the  influence  of  the  air  and  sunlight,  forming 
carbonic  acid.  These  reactions  are  purely  chemical,  taking  place  without  the 
intervention  of  microbes,  and  are  accompanied  by  a  change  in  color  of  the  orig- 
inal humus.  The  oxidation  is  rendered  more  active  through  the  division  and 
mellowing  of  the  humus  by  cultivation.  Through  chemical  union  of  the  carbonic 
acid  with  certain  bases,  as  lime  soda  and  potash,  there  are  found  soluble  car- 
bonates which  may  be  leached  out  by  meteoric  waters. 

1  Geikie,  Text-book  of  Geology,  3d  ed.,  p.  472. 

The  writer  was  shown  not  long  since,  by  Professor  Charles  E.  Munroe,  a 
very  practical  illustration  of  the  remarkable  corrosive  power  of  organic  acids. 
A  highly  ornate  Erench  clock,  with  case  of  black  marble,  was  packed  for  storage 
in  excelsior  which  was  a  trifle  damp.  The  clock  remained  in  storage  from  the 
last  of  May  until  about  the  first  of  October  of  the  same  year.  When  the  pack- 
ing material  was  removed,  the  marble  was  found  to  be  so  corroded  as  to  need 
rehoning  and  polishing.  The  roughness  could  be  easily  felt  by  passing  the 
finger  over  the  surface,  and  long  lustreless  lines  indicating  the  contact  of  excel- 
sior fibres  traversed  the  surface  in  every  direction. 

2  Julien,  The  Geological  Action  of  Humus  Acids,  Proc.  Am.  Assoc.  Adv.  of 
Science,  1879,  p.  324. 


CHEMICAL   ACTION   OF    WATER  191 

It  is  stated  by  Storer1  that  "on  the  tops  of  the  higher  hills 
of  New  Hampshire,  and  on  the  coast  of  Maine  also,  a  cold,  sour 
black  earth  will  often  be  noticed  at  the  surface  of  the  ground, 
immediately  beneath  which  is  sometimes  a  layer  of  remarkably 
white  earth.  The  whiteness  is  due  to  the  solvent  action  of 
acids  that  soak  out  from  the  black  humus,  and  which  leach  out 
from  the  underlying  clay  and  sand  the  oxides  of  iron  that  for- 
merly colored  them." 

As  long  ago  as  1848  the  Rogers  brothers  showed2  that  pure 
water  partially  decomposed  nearly  all  the  ordinary  silicate 
minerals  which  form  any  appreciable  part  of  our  rocks.  The 
action  of  carbonated  water  was  recognizable  in  less  than  ten 
minutes,  but  pure  water  required  a  much  longer  time  before 
its  effect  was  sufficient  for  a  qualitative  determination.  So  pro- 
nounced was  the  action  of  carbonated  water  that  the  presence  of 
the  alkalies  of  lime  and  magnesia  could  be  recognized  in  a  single 
drop  of  the  nitrate  from  the  liquid  in  which  the  powdered  min- 
erals were  digested.  By  digestion  for  forty-eight  hours  they 
obtained  from  hornblende,  actinolite,  epidote,  chlorite,  serpen- 
tine, feldspar,  etc.,  a  quantity  of  lime,  magnesia,  oxide  of  iron, 
alumina,  silica,  and  alkalies  amounting  to  from  0.4  %  to  1  %  of 
the  whole  mass.  The  lime,  magnesia,  and  alkalies  were  ob- 
tained in  the  form  of  carbonates ;  the  iron,  in  the  case  of  horn- 
blende, epidote,  etc.,  passing  from  the  state  of  carbonate  to  that 
of  peroxide  during  the  evaporation  of  the  solutions.  Forty 
grains  of  finely  pulverized  hornblende,  digested  for  forty- 
eight  hours  in  carbonated  water  at  a  temperature  of  60°,  with 
repeated  agitation,  yielded  —  silica,  0.08%;  oxide  of  iron, 
0.095%;  lime,  0.13%,  and  magnesia,  0.095%,  with  traces  of 
manganese.  Commenting  on  these  results,  Bischof  remarks3 
that  "by  repeating  this  treatment  112  times  with  fresh  carbon- 
ated water,  a  perfect  solution  might  be  effected  in  224  days. 
If  now,"  he  says,  "40  grains  of  hornblende,  unpowdered,  in 
which,  according  to  the  above  assumption,  the  surface  is  only 
one  millionth  of  the  powdered,  were  treated  in  the  same  way, 
and  the  water  renewed  every  two  days,  the  time  required  for 
perfect  solution  would  be  somewhat  more  than  six  million 
years."  In  considering  these  figures  and  their  practical  bear- 

1  Chemistry  as  applied  to  Agriculture. 

2  Am.  Jour,  of  Science,  Vol.  V,  1848. 

3  Chemical  and  Physical  Geology,  Vol.  I,  p.  61. 


192       THE   PRINCIPLES   INVOLVED   IN   ROCK- WEATHERING 


ing,  it  must  be  remembered  that  while  in  nature  the  quantity 
of  water  coming  in  contact  with  a  crystal  embedded  in  a  rock 
during  a  given  time  is  much  less  than  that  assumed  above,  the 
mineral  is  undergoing  a  gradual  splitting  up,  becoming  more 
and  more  porous,  so  that  the  process  is  gradually  accelerated. 

To  quote  Bischof  again,  it  is  probably  admissible  to  assume 
that  the  time  in  which  water  produces  similar  effects  of  decom- 
position or  solution  on  minerals,  is  inversely  as  the  magnitude 
of  the  surface  of  contact.  If,  therefore,  a  mineral  were  so  far 
subdivided  that  the  surface  was  increased  ten  million-fold,  the 
quantity  then  dissolved  during  a  certain  time  would  be  the  same 
as  that  dissolved  during  a  period  ten  million  times  as  long. 

Richard  Miiller  has  also  shown1  that  carbonic  acid  waters 
will  act  even  during  so  brief  a  period  as  seven  weeks  upon  the 
silicate  mineral  with  such  energy  as  to  permit  a  quantitative 
determination  of  the  dissolved  materials.  The  accompanying 
table  shows  (1st)  the  percentages  of  the  various  constituents 
thus  taken  out  by  the  carbonated  water,  and  (2d)  the  total  per- 
centages of  the  materials  dissolved.  That  is  to  say,  the  figures 
0.1552  given  for  adular  under  SiO2,  indicate  that  0.1552%  of 
the  total  65.24%  of  the  silica  contained  by  the  mineral  have 
been  removed,  and  so  on.  The  last  column  gives  the  total  per 
cent  of  all  the  constituents  extracted. 


MINERAL 

Si02 

A1203 

K20 

Na2O 

MgO 

CaO 

P»0, 

FeO 

Total 

Adular  . 

0  1552 

01368 

% 

% 

% 

trace 

0328 

Oligoclase  .    . 
Hornblende    . 

0.237 
0.419 

9.1713 
trace 

.... 

2.367 

.... 

3.213 

8528 

.... 

trace 
4.829 

0.533 
1.536 

Magnetite  .     . 

trace 

0.942 

0.307 

Apatite  . 

2  168 

1  822 

2018 

Olivine  .     .     . 
Serpentine 

0.873 
0.354 

trace 

.... 

.... 

1.291 
2649 

trace 

8.733 
1.527 

2.111 
1.211 

The  summary  of  his  investigations  he  gives  as  below :  — 

(1)  All  the  minerals  tested  were  acted  upon  by  the  carbonated 

water. 

(2)  In  this  process  there  were  formed  carbonates  of  lime,  iron, 

manganese,  cobalt,  nickel,  potash,  and  soda. 

1  Untersuchen  liber  die  Einwirkung  des  kohlensaurehaltigen   Wassers  auf 
einige  Mineralien  und  Gesteine,  Tschermaks  Min.  Mittheilungen,  1877,  p.  25. 


PLATE    15 


v^v^iSSl 

•> »,       ««£*  J 
,    -.  ^      -^--r. 


Corroded  limestones. 


CHEMICAL  ACTION    OF    WATER  193 

(3)  In  the  action  of  the  carbonated  waters  upon  the  alkaline 

silicates,  like  the  feldspars,  a  small  amount  of  silica  went 
always  into  solution,  presumably  in  the  form  of  hydrate. 

(4)  Even  alumina  was  dissolved  in  appreciable  quantities. 

(5)  Adular  proved  more  resisting  to  the  action  of  the  acid  than 

did  the  oligoclase. 

(6)  The  first  stage  of  decomposition  in  the  feldspars  is  a  redden- 

ing process ;  the  second,  kaolinization. 

(7)  Hornblende  was  more  easily  decomposed  than  feldspar. 

(8)  Increase  of  pressure  on  the  solution  was  productive  of  more 

energetic  action  than  prolonging  the  time. 

(9)  Of   all   the   minerals   tested,  the   magnetic  iron   was   least 

affected. 

(10)  Apatite  was  readily  acted  upon,  as  could  be  detected  by  its 

appearance  under  the  microscope. 

(11)  Olivine  was  the  most  readily  attacked  of  all  the  silicates 

tested,   probably   twice   as   easily  decomposed   as  the  ser- 
pentine. 

(12)  Magnesian  silicates  were  attacked  by  the  carbonated  waters. 

Hence  serpentine  cannot  be  considered  a  final  product  of 
decomposition.1 

Of  all  the  materials  forming  any  essential  part  of  the  earth's 
crust  the  limestones  are  most  affected  by  the  solvent  power  of 
water.  It  is  stated  that  pure  water  will  dissolve  lime  carbon- 
ate in  the  proportions  of  one  part  in  10800  when  cold  and  one 
part  in  8875  when  boiling. 

Since  rock-weathering  is,  as  already  stated,  a  superficial 
phenomenon,  we  have  to  do  only  with  waters  of  ordinary  tem- 
peratures and  under  ordinary  conditions  of  pressure,  though 
this  expression  must  not  be  taken  as  necessarily  meaning  cold 
waters,  since,  if  we  accept  the  statements  of  Caldcleugh,2  rain 
waters  falling  upon  the  heated  rocks  may  have  their  tempera- 
tures raised  as  high  as  140°  F.  The  enormously  destructive 
effect  of  carbonated  waters  on  limestone  is  scarcely  apparent 
011  casual  inspection,  owing  to  the  fact  that  the  material  is 
carried  away  in  solution,  leaving  only  the  insoluble  impurities 
behind.  In  such  cases  it  is  possible  to  estimate  the  amount  of 
corrosion  through  a  comparison  of  the  proportional  amounts  of 
various  constituents  in  this  residue  with  those  in  the  fresh  rock 

1  Serpentine,  however,  cannot  be  properly  considered  a  decomposition  prod- 
uct.    It  is  rather  a  product  of  alteration. 

2  Trans.  Geol.  Soc.  of  London,  1829. 


194        THE   PRINCIPLES  INVOLVED   IN  ROCK-WEATHERING 

(see  p.  209  et  seq.),  and  the  time  limit  of  corrosion  through 
determining  the  percentage  amounts  of  the  constituents  in  the 
water  which  annually  drains  from  any  given  area.  By  such 
methods  it  has  been  estimated1  that  some  275  tons  of  calcium 
carbonate  are  annually  removed  from  each  square  mile  of  Cal- 
ciferous  limestone  exposed  in  the  Appalachian  region  alone  ; 
while  a  well-known  English  authority2  has  calculated  that  with 
an  annual  rainfall  of  32  inches,  percolating  only  to  a  depth  of 
18.3  inches,  there  are  annually  removed  by  solution  from  the 
superficial  portions  of  England  and  Wales  an  average  of  all 
constituents  amounting  to  143.5  tons  per  square  mile  of  area. 
He  further  calculates  that  the  average  amount  of  carbonate  of 
lime  annually  removed  from  each  square  mile  of  the  entire 
globe  amounts  to  50  tons.3  It  is  to  this  corrosive  action  of 
meteoric  waters  that  still  another  authority4  would  attribute 
the  slight  thickness  and  nodular  condition  of  many  beds  of 
Palseozoic  limestone.  He  argues  that  originally  thick-bedded 
limestones  have,  during  the  ages  subsequent  to  their  formation 
and  uplifting,  become  so  impoverished  through  the  dissolving 
out  and  carrying  away  in  solution  of  the  lime  carbonate,  as  to 
have  been  quite  obliterated,  or  reduced  to  mere  nodular  bands, 
and  given  rise  to  important  palseontological  breaks  in  the  geo- 
logical record.  Other  than  organic  acids  may  locally  exert  a 
potent  influence.  Thus  Robert  Bell  has  described  the  dolomitic 
limestones  underlying  the  waters  along  Grand  Manitou  Island, 
the  Indian  peninsula,  and  adjacent  portions  of  Lake  Huron  and 
the  Georgian  Bay,  as  pitted  and  honeycombed  in  a  very  pecu- 
liar and  striking  manner.  This  corrosion,  it  is  believed,  is 
produced  through  the  solvent  action  of  sulphuric  acid  in  the 
water,  the  acid  itself  arising  from  the  decomposition  of  the  sul- 
phides of  iron,  pyrites  and  pyrrhotite,  which  exist  in  great 
quantities  in  the  Huronian  rocks  to  the  northward.5 

1  A.  L.  Ewing,  Am.  Jour,  of  Science,  1885,  p.  29. 

2  T.  Mellard  Reade,  Chemical  Denudation  in  Relation  to  Geological  Time. 

3  The  total  dissolved  constituents  thus  removed  are  divided  up  as  follows : 
Carbonate  of  lime,  50  tons  ;  sulphate  of  lime,  20  tons  ;  silica,  7  tons  ;  carbonate 
of  magnesia,  4  tons ;  peroxide  of  iron,  1  ton ;  chloride  of  sodium,  8  tons ;  alka- 
line carbonates  and  sulphates,  6  tons. 

4  F.  Rutley,  The  Dwindling  and  Disappearance  of  Limestones,  Quar.  Jour. 
Geol.  Soc.  of  London,  August,  1893. 

5  Bull.  Geol.  Soc.  of  America,  Vol.  VI,  pp.  47-304. 

Messrs.  C.  \V.  Hayes  and  M.  R.  Campbell,  of  the  United  States  Geological 


MECHANICAL  ACTION   OF   WATER^AND   OF   ICE  195 


3.     MECHANICAL  ACTION  OF  WATER  AND  OF  ICE 

Aside  from  its  solvent  capacity,  water  acts  as  a  powerful  ero- 
sive agent,  as  well  as  an  agent  for  the  transportation  of  the 
eroded  materials.  It  is  only  its  erosive  power  that  need  con- 
cern us  here,  though,  as  will  be  seen,  this  is  to  a  considerable 
extent  dependent  upon  its  power  of  transportation.  Every 
raindrop  beating  down  upon  a  surface  already  sorely  tried  by 
heat  and  frost  serves  to  detach  the  partially  loosened  granules, 
and,  catching  them  up  in  the  temporary  rivulets,  carries  them 
to  the  more  permanent  rills,  to  be  spread  out  over  the  valley 
bottoms,  or  perhaps  if  the  slopes  be  steep  and  the  current  ac- 

Survey,  have  recently  reported  some  remarkable  examples  of  corroded  quartz 
pebbles  which  should  be  mentioned  here,  although  a  satisfactory  explanation  for 
the  phenomenon  has  not  yet  been  given. 

Dr.  Hayes,  in  a  personal  memorandum  to  the  writer,  describes  the  occur- 
rence as  follows:  — 

"At  three  rather  widely  separated  points  in  the  South,  conglomerates  have 
been  observed  in  which  the  projecting  portions  of  the  pebbles  have  been  etched 
or  partly  dissolved. 

"The  first,  observed  by  Mr.  Campbell,  is  at  Nuttall,  West  Virginia.  The 
conglomerate  in  question,  which  belongs  to  the  coal  measures,  is  composed  of 
rather  coarse  quartz  sand  with  slightly  yellowish  cement,  in  which  are  embedded 
well-worn  pebbles  of  white  vein  quartz.  The  latter  vary  in  size  up  to  three- 
quarters  of  an  inch  in  diameter,  and  are  somewhat  irregularly  distributed. 
Ordinarily  the  pebbles,  wholly  unaltered,  weather  out  by  the  chemical  or 
mechanical  disintegration  of  the  sandy  matrix.  In  the  case  observed,  however, 
where  the  conglomerate  received  the  drip  from  an  overhanging  cliff,  the  project- 
ing portions  of  the  pebbles  are  deeply  pitted,  evidently  by  solution.  Mechanical 
wear  is  precluded  by  the  form  of  the  resulting  surface,  which  is  not  smooth  like 
the  portions  of  the  pebble  still  protected  by  the  matrix,  but  is  rough  and  irregu- 
lar. The  outer  portion  of  the  pebbles  is  evidently  less  easily  affected  by  the 
solvent  than  the  interior,  and  forms  a  sharp  rim  about  the  irregular  cavities 
hollowed  out  within.  In  some  cases  a  third  of  the  pebble  has  thus  been  re- 
moved. The  surface  of  the  sandstone  matrix  in  which  the  pebbles  are  embedded 
is  also  pitted,  possibly  by  the  same  process  of  solution  as  that  which  has  affected 
the  pebbles,  but  such  a  surface  might  also  be  produced  by  mechanical  means  in 
case  the  cement  were  less  indurated  in  some  places  than  in  others. 

"The  second  case  is  on  Clifty  Creek,  White  County,  Tennessee.  The  con- 
glomerate, also  a  member  of  the  coal  measures,  forms  the  bottom  of  a  small 
canon,  and  is  covered  by  the  creek  at  high  water,  but  uncovered  throughout 
the  greater  part  of  the  year.  The  matrix  is  a  coarse  white  sandstone  which 
weathers  yellow  by  the  oxidation  of  the  slightly  ferruginous  cement.  Embedded 
in  this  are  rather  abundant  pebbles,  varying  in  size  up  to  two  inches  in  diameter, 
and  composed  chiefly  of  quartz,  with  a  few  of  chert  and  possibly  of  quartzite. 
The  projecting  portions  of  these  pebbles  have  been  in  part  removed,  though  they 
still  project  somewhat  above  the  enclosing  matrix.  As  in  case  of  the  Nuttall 
conglomerate,  the  exterior  portions  of  the  pebbles  are  less  easily  affected  than 


196        THE   PRINCIPLES  INVOLVED   IN   ROCK- WEATHERING 

cordingly  strong,  to  the  rivers  and  thence  to  the  sea.  The 
amount  of  detrital  matter  thus  mechanically  removed  from 
the  hills  and  spread  out  over  valley  and  sea-bottoms  quite  ex- 
ceeds our  comprehension,  but  it  is  estimated  that  at  the  rate 
the  Mississippi  River  is  now  doing  its  work,  the  entire  Ameri- 
can continent  might  be  reduced  to  sea-level  within  a  period  of 
four  and  one-half  million  years.  The  Appalachian  Mountain 
system,  whose  uplifting  began  in  early  Cambrian  times  and 
terminated  at  the  close  of  the  Carboniferous,  has  already 
through  this  cause  lost  more  material  than  the  entire  mass  of 
that  which  now  remains.  But  the  rivers,  like  the  winds  arid 
glaciers,  in  virtue  of  this  load  they  bear,  become  themselves 
converted  into  agents  of  erosion,  filing  away  upon  their  rocky 
beds,  undermining  their  banks,  and  continually  wearing  away 
the  land  by  their  ceaseless  activity.  The  pot-holes  in  the  bed 
of  a  stream,  formed  by  the  constant  swirl  of  sand  and  gravel 
in  an  eddy,  furnish  on  a  small  scale  striking  illustrations  of  this 
cutting  power,  while  the  rocky  canons  o'f  the  Colorado  of  the 
West,  where  thousands  of  feet  of  horizontal  strata  have  been  cut 
through  as  with  a  file,  show  the  same  thing  on  a  scale  so  gigan- 
tic as  to  be  at  first  scarce  comprehensible.1  An  item  of  no 
insignificant  importance  to  be  considered  here  is  the  possibility, 

the  interiors,  and  when  the  pebble  has  been  a  third  or  half  removed  the  outer 
shell  forms  a  rim  within  which  is  a  depression  with  a  slight  elevation  in  the 
centre.  The  chert  pebbles  show  less  evidence  of  corrosion  by  a  solvent  than 
those  composed  of  quartz.  Their  upper  surfaces  are  somewhat  worn  down  and 
even  slightly  hollowed,  but  this  might  easily  have  been  produced  by  mechanical 
means,  which  is  not  the  case  with  the  quartz. 

"  The  third  case  is  a  block  of  conglomerate  from  Starrs  Mountain,  Tennessee, 
collected  by  Mr.  Bailey  Willis.  This  is  of  Lower  Cambrian  age.  The  matrix  is 
a  coarse  feldspathic  sandstone  containing  layers  of  well-rounded  pebbles,  mostly 
quartz,  with  a  few  probably  of  some  feldspar.  The  former  are  between  one-half 
and  one  inch  in  diameter  and  the  latter  somewhat  larger.  The  projecting  por- 
tions of  the  quartz  pebbles  on  one  side  of  the  block  are  almost  entirely  removed, 
and  as  in  the  other  cases  evidently  by  solution.  A  slight  rim  projects  above  the 
matrix  in  which  the  pebbles  are  embedded  ;  within  this  is  a  depression,  while  a 
slight  elevation  occupies  the  centre. 

"The  projecting  portions  of  the  feldspathic  pebbles  also  are  partly  removed, 
but  this  may  be  due  to  corrasion  instead  of  corrosion,  that  is,  to  the  action  of 
mechanical  rather  than  chemical  agents.  The  pebbles  on  the  lower  side  of  the 
block  have  their  original  water- worn  surfaces  without  any  trace  of  etching." 

1  Captain  C.  E.  Button  has  estimated  (Tertiary  History  of  the  Grand  Canon 
of  the  Colorado)  that  from  over  an  area  of  13,000  to  15,000  square  miles  drained 
by  the  Colorado  River,  an  average  thickness  of  10,000  feet  of  strata  have  been 
removed. 


MECHANICAL  ACTION   OF   WATER  AND   OF   ICE  197 

indeed  probability,  of  an  incidental  chemical  decomposition 
taking  place  during  this  abrasive  action.  Daubree  showed  l 
that  when  feldspathic  fragments  were  submitted  to  artificial 
trituration  in  a  revolving  cylinder  containing  water,  a  decompo- 
sition was  effected  whereby  the  alkalies  were  liberated  in  very 
appreciable  amounts.  He  found  further  that  the  principal 
product  of  mutual  attrition  of  feldspar  fragments  in  water  was 
not  sand,  but  an  impalpable  mud  (limon).  This  mud  was  of 
such  tenuity  as  to  remain  for  many  days  in  suspension,  and 
on  desiccation  became  so  hard  as  to  be  broken  only  with 
the  aid  of  a  hammer,  resembling  in  many  respects  the  argillites 
of  the  coal  measures,  but  differing  in  that  it  carried  a  high 
percentage  of  alkalies.  Granitic  rocks  thus  treated  yielded 
angular  fragments  of  quartz  and  very  minute  shreds  of  mica, 
while  the  feldspars  ultimately  quite  disappeared  in  the  form 
of  the  impalpable  mud  above  mentioned.  It  was  noted  that 
after  the  quartzose  particles  had  reached  a  certain  degree  of 
fineness  further  diminution  in  the  size  ceased,  owing  to  the 
buoyant  action  of  the  water,  which  in  the  form  of  a  thin  film 
between  adjacent  particles  acted  as  a  cushion  and  prevented 
actual  contact  to  the  extent  necessary  for  mutual  abrasion.  It 
is  to  a  similar  action  on  the  part  of  sea-water  that  Shaler  2  would 
attribute  the  lasting  qualities  of  the  sand  grains  upon  our  sea 
beaches.  Indeed  the  conditions  of  Daubree's  experiments  as 
a  whole  were  not  so  different  from  those  existing  in  nature  that 
we  need  hesitate,  as  it  seems  to  the  writer,  to  conclude  similar 
action,  both  chemical  and  physical,  may  be  going  on  wherever 
abrasion  takes  place  in  the  presence  of  continual  moisture,  as  in 
the  bed  of  a  river  or  glacier. 

1  It  will  be  remembered  that  this  authority  placed  rock  fragments  in  stone  and 
iron  cylinders  containing  water  and  made  to  revolve  horizontally  at  a  measured 
rate  of  speed,  so  that  the  actual  distance  travelled  by  any  of  the  particles  dur- 
ing a  given  time  could  be  readily  calculated.    The  product  of  this  disintegration, 
even  when  carried  to  the  condition  of  fine  silt,  was  always  sharply  angular.     His 
experiments  further  showed  that  when  feldspathic  fragments  were  thus  treated, 
there  was  always  a  certain  amount  of  decomposition,  whereby  salts  of  potash  were 
liberated  ;  in  one  instance,  when  3  kilogrammes  of  feldspar  were  revolved  for  192 
hours  in  iron  cylinders  containing  5  litres  of  water,  2.72  kilogrammes  of  finely 
comminuted  mud  were  obtained,  and  in  solution  in  the  water,  12.6  grammes  of 
potash,  or  2.52  grammes  per  litre.     The  presence  of  carbonic  acid  in  the  water 
increased  the  amount  of  potash.    When  the  feldspar  was  triturated  dry  and  then 
treated  with  water,  no  such  solvent  action  could  be  detected.  —  Geologic  Experi- 
mental, p.  268. 

2  Bull.  Geol.  Soc.  of  America,  Vol.  V,  p.  208. 


198       THE  PRINCIPLES   INVOLVED   IN  ROCK- WEATHERING 

The  hammering  action  of  waves  upon  the  sea-coast  exerts  a 
powerful  erosive  action,  particularly  upon  particles  of  rock  of 
such  size  as  to  be  lifted  or  moved  by  wave  action,  but  too  heavy 
to  be  protected  from  attrition  by  the  thin  film  of  water  above 
alluded  to.  Shaler's  observations1  at  Cape  Ann  were  to  the 
effect  that  ordinary  granitic  paving  blocks  (weighing  perhaps 
twenty  pounds)  were,  when  exposed  to  surf  action,  worn  in 
the  course  of  a  year  into  spheroidal  forms  such  as  to  indicate 
an  average  loss  of  more  than  an  inch  from  their  peripheries. 
Experiments  made  with  fragments  of  hard  burned  brick  showed 
that  in  the  course  of  a  year  they  would  be  reduced  fully  one-half 
their  bulk.  Even  the  crystallization  of  the  salt  thrown  up  by 
wave  action  and  absorbed  into  the  pores  of  rocks  2  serves  in  its 
way  the  purposes  of  disintegration. 

The  Action  of  Freezing  Water  and  of  Ice.  —  The  action  of 
dry  heat  and  cold  in  disintegrating  rocks  has  already  been 
described.  The  effects  of  such  temperature  changes  upon 
stone  of  ordinary  dryness  are,  however,  slight  in  comparison 
with  the  destructive  agencies  of  freezing  temperatures  upon 
stones  saturated  with  moisture.  The  expansive  force  of  water 
passing  from  the  liquid  to  the  solid  state  has  been  graphically 
described  as  equal  to  the  weight  of  a  column  of  ice  a  mile  high 
(about  150  tons  to  the  square  foot).  Otherwise  expressed,  100 
volumes  of  water  expand,  on  freezing,  to  form  109  volumes  of 
ice.  Provided,  then,  sufficient  water  be  contained  within  the 
pores  of  a  stone,  it  is  easy  to  understand  that  the  results  of 
freezing  must  be  disastrous.  That  stones  as  they  lie  in  the 
ground  do  contain  moisture,  often  in  no  inconsiderable  amounts, 
is  a  well-known  and  well-recognized  fact  by  all  those  engaged 
in  quarrying  operations,  and  indeed  no  mineral  substance  is 
absolutely  impervious  to  it.  The  amount  contained,  naturally 
varies  with  the  nature  of  the  mineral  constituents  and  their 
state  of  aggregation.  According  to  various  authorities,  granite 
may  contain  some  0.37%  by  weight;  chalk,  20%;  ordinary 
compact  limestone,  0.5%  to  5%  ;  marble,  about  0.30%  ;  and 
sandstones,  amounts  varying  up  to  10%  or  12%,  while  clay 

1  Bull.  Geol.  Soc.  of  America,  Vol.  V,  p.  208. 

2  According  to  Dana  (Wilkes'  Exploring  Expedition,  Geology,  p.  529),  the 
sandstones  along  the  coast  of  Sydney,  Australia,  are  subjected  to  a  mechanical 
disintegration  through  the  crystallization  of  salt  which  is  absorbed  from  the 
saline  spray  of  the  ocean  waves. 


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MECHANICAL   ACTION  OF   WATER   AND   OF   ICE  199 

may  contain  nearly  one-fourth  its  weight.  This  water  is  largely 
interstitial  —  the  quarry  water,  as  it  is  sometimes  called.  In 
addition  to  this,  the  quartz,  particularly  of  granitic  rocks, 
almost  universally  contains  innumerable  minute  cavities  par- 
tially filled  with  water,  and  which  are,  in  extreme  cases,  so 
abundant  as  to  make  up,  according  to  Sorby,  at  least  5  %  of  the 
whole  volume  of  the  mineral. 

That  the  passage  of  this  included  moisture  from  the  liquid 
to  the  solid  state,  must  be  attended  with  results  disastrous  to 
the  stone  is  self-evident,  though  the  rate  of  disintegration  may 
be  so  slow  under  favorable  circumstances  as  to  be  scarce  notice- 
able. Freezing  of  the  absorbed  water  is  one  of  the  most  fruit- 
ful sources  of  disintegration  in  stones  confined  in  the  walls  of 
a  building,  and  even  in  the  quarry  bed  it  is  by  no  means  uncom- 
mon to  have  stone  so  injured  as  to  render  it  worthless.  How- 
ever slight  may  be  the  effects  of  a  single  freezing,  constant 
repetition  of  the  process  cannot  fail  to  open  up  new  rifts,  and 
still  further  widen  those  already  in  existence,  allowing  further 
penetration  of  water  to  freeze  in  its  turn  and  to  exert  a  chemical 
action  as  well.,  So  year  in  and  year  out,  through  winter's  cold 
and  summer's  heat,  the  work  goes  on  until  the  massive  rock 
becomes  loose  sand  to  be  caught  up  by  winds  or  temporary 
rivulets  and  spread  broadcast  over  the  land.  In  some  instances, 
it  may  be,  the  rock  is  of  sufficiently  uniform  texture  to  be  af- 
fected in  all  its  mass  alike.  More  commonly,  however,  it  is 
traversed  by  veins,  joints,  or  other  lines  of  weakness  along 
which  the  rifting  power  is  first  made  manifest,  as  in  our  illus- 
tration. Naturally  disintegration  of  this  kind  is  confined  to 
frigid  and  temperate  latitudes.  As  bearing  upon  the  extreme 
rapidity  with  which  such  disintegration  may  take  place,  the 
following  is  quoted  from  a  letter  of  Dr.  L.  Stejneger,  of  the 
United  States  National  Museum,  who  passed  several  months 
among  the  islands  of  Bering  Sea. 

uln  September,  1882,  I  visited  Tolstoi  Mys,  a  precipitous 
cliff  near  the  southeastern  extremity  of  Bering  Island.  At  the 
foot  of  it  I  found  large  masses  of  rock  and  stone  which  had 
evidently  fallen  down  during  the  year.  Most  of  them  were 
considerably  more  than  six  feet  in  diameter,  and  showed  no 
trace  of  disintegration.  The  following  spring,  April,  1883, 
when  I  revisited  the  place,  I  found  that  the  rocks  had  split  up 
into  innumerable  fragments,  cube-shaped,  sharp-edged,  and  of 


200       THE   PRINCIPLES  INVOLVED   IN   ROCK-WEATHERING 

a  very  uniform  size,  —  about  two  inches.  They  had  not  yet 
fallen  to  pieces,  the  rocks  still  retaining  their  original  shape. 
I  may  remark,  however,  that  the  weather  was  still  freezing 
when  I  was  there.  The  winter  was  not  one  of  great  severity, 
and  several  thawing  spells  broke  its  continuity.  These  cubic 
fragments  did  not  seem  to  split  up  any  further,  for  everywhere 
on  the  islands  where  the  rock  consisted  of  the  coarse  sandstone, 
as  in  this  place,  the  talus  consisted  of  these  sharp-edged  stones." 

Ice  acts  as  a  disintegrating  agent  in  still  other  ways  than 
that  mentioned.  The  phenomenon  of  the  glacier  is  now  so 
well  known  that  we  need  dwell  upon  it  but  briefly  here.  Long- 
continued  precipitation  of  snow  upon  regions  of  such  elevation, 
or  in  such  latitudes  as  to  preclude  anything  like  an  equally 
rapid  melting,  gives  rise  to  deep  fields  of  snow,  compacted  in 
the  lower  portions  into  the  condition  of  ice.  These,  in  virtue 
of  the  weight  of  the  overlying  mass,  and  perhaps  the  steepness 
of  the  slopes,  aided  by  a  certain  amount  of  plasticity  possessed 
in  some  degree  by  even  the  most  rigid  of  so-called  solids,  creep 
slowly  down  the  slopes  in  the  form  of  glaciers  or  rivers  of  ice. 
Advancing,  it  may  be,  but  an  inch  or  several  feet  a  day,  now  scarce 
moving  at  all,  or  even  retreating  temporarily  through  a  diminu- 
tion in  the  amount  of  their  supplies,  or  an  increase  in  the  sun's 
heat,  these  bring,  either  upon  their  surfaces  as  moraines,  or 
frozen  into  their  mass,  large  quantities  of  fragmental  rock  mate- 
rial fallen  upon  them  from  above,  or  picked  up  from  the  surfaces 
over  which  they  flow.  Those  fragments  which  remain  upon  the 
upper  surface,  or  frozen  into  the  upper  portions,  are  but  trans- 
ported to  the  lower  levels  where,  the  temperature  being  suffi- 
cient, the  ice  is  melted  and  the  load  deposited  in  the  form  of  a 
moraine. 

Beneath,  and  frozen  into  the  lower  portion  of  the  ice  sheet, 
there  is,  however,  a  variable  amount  of  rock  material,  which,  as 
the  glacier  moves  along,  is  crowded  with  all  the  weight  of  the 
overlying  mass,  and  all  the  resistless  energy  of  the  ice  behind, 
over  the  surface  of  the  underlying  rock.  In  virtue  of  this 
material,  this  sand,  gravel,  and  boulder  aggregate,  the  glaciers 
become  converted  into  what  we  may  compare  to  extremely 
coarse  files,  to  tear  away  the  rocks  over  which  they  pass,  and 
grind  and  crush  them  into  detritus  of  varying  degrees  of 
fineness.  The  small  streams  which  originate  from  the  melt- 
ing of  these  glaciers  become,  hence,  not  infrequently  charged 


ACTION   OF   PLANTS    AND   ANIMALS  201 

to  the  point  of  turbidity  with  the  fine  silt-like  detritus  ground 
from  the  ledges  and  in  part  from  the  boulders  themselves. 
Figure  3  of  plate  19  shows  a  slab  of  limestone  still  bear- 
ing upon  its  surface  the  evidences  of  the  severity  of  the 
onslaught.  A  consideration  of  the  amount  of  detritus  thus 
brought  down  either  merely  as  transported  or  as  abraded 
material  belongs  properly  to  the  chapter  on  transportation, 
but  a  few  illustrations  are  not  without  interest  here.  The 
Aar  in  Switzerland  is  stated  by  Geikie  to  discharge  every  day 
in  August  some  440,000,000  gallons  of  water,  carrying  some 
280  tons  of  sand.  A  portion  of  this  is  in  a  state  of  such 
minute  subdivision  as  to  remain  a  long  time  in  suspension, 
and  give  the  water  a  milky  appearance  for  several  miles. 
I.  C.  Russell  has  described1  the  Tuolumne  River,  issuing  from 
the  foot  of  the  Lyell  Glacier  in  the  Sierras  of  California,  as 
turbid  with  silt  which  has  been  ground  by  the  moving  ice. 

At  the  foot  of  the  Dana  Glacier  there  is  a  small  lakelet 
whose  waters  are  of  a  peculiar  greenish  yellow  color  from 
the  silt  held  in  suspension,  and  which,  when  submitted  to 
microscopic  examination,  is  found  to  be  made  up  of  fresh 
angular  fragments  of  various  silicate  minerals  of  all  sizes  from 
0.35  mm.  in  diameter  down  to  impalpable  silt. 

4.     ACTION  OF   PLANTS  AND  ANIMALS 

Both  plants  and  animals  aid  to  some  extent  in  the  work  of 
rock  disintegration.  Plants  are  also  not  infrequently  an  im- 
portant factor  in  promoting  sedimentation,  while  burrowing 
insects  and  animals  may  exert  an  important  influence  upon 
the  texture  of  soils  and  in  bringing  about  a  more  general 
admixture  by  transferring  to  the  surface  that  which  is  below. 

The  lowest  forms  of  plant  life,  —  the  lichens  and  mosses,  - 
growing  upon  the  hard,  bare  face  of  rocky  ledges  send  their 
minute   rootlets    into   every   crack   and    crevice,    seeking   not 
merely  foot-hold,  but  food  as  well. 

Slight  as  is  the  action,  it  aids  in  disintegration.  The  plants 
die,  and  others  grow  upon  their  ruins.  There  accumulates 
thus,  it  may  be  with  extreme  slowness,  a  thin  film  of  humus, 
which  serves  not  merely  to  retain  the  moisture  of  rains  and 
thus  bring  the  rock  under  the  influence  of  chemical  action, 

1 5th  Ann.  Rep.  U.  S.  Geol.  Survey,  1883-84. 


202        THE   PRINCIPLES   INVOLVED    IN   ROCK- WEATHERING 

but  supplies  at  the  same  time  small  quantities  of  the  humic 
and  other  organic  acids  to  which  reference  has  already  been 
made.1  These  act  both  as  solvents  and  deoxidizing  agents. 
As  time  goes  on,  sufficient  soil  gathers  for  other,  larger  and 
higher  types  of  life,  which  exert  still  more  potent  influences. 
It  may  be  the  rock  is  in  a  jointed  condition.  Into  these  joints 
each  herb,  shrub,  or  sapling  pushes  down  its  roots,  which,  in 
simple  virtue  of  their  gain  in  bulk,  day  by  day,  serve  to  enlarge 
the  rifts  and  furnish  thereby  more  ready  access  for  water,  and 
the  wash  of  rains,  to  still  further  augment  disintegration. 

This  phase  of  root  action  is  often  well  shown  in  walls  of 
ancient  masonry,  either  of  brick  or  stone,  whereby  the  usual 
rate  of  destruction  is  greatly  accelerated.  The  depth  to  which 
such  roots  may  penetrate  has  often  been  noted,  varying,  as  is 
to  be  expected,  with  the  nature  of  the  soil.2  In  the  limestone 
caverns  of  the  Southern  states,  the  writer  has  often  been  im- 
pressed by  the  number  of  long  thread-like  rootlets,  so  fine  as 
to  be  almost  imperceptible,  which  have  found  their  way  through 
rifts  in  the  rocky  roof. 

H.  Carrington  Bolton  has  shown  that  very  many  minerals 
are  decomposed  by  the  action  of  cold  citric  acid  for  a  more  or 
less  prolonged  period,  the  zeolites  and  other  hydrous  silicates 
being  especially  susceptible.  Such  tests  have  a  peculiar  sig- 
nificance when  we  consider  that  the  roots  of  growing  plants 
secrete  an  acid  sap,  which,  by  actual  experiment,  has  been  found 
capable  of  etching  marble.  The  exact  nature  of  this  acid  is 
not  accurately  known,  but  it  is  considered  probable  that  in  the 
rootlets  of  each  species  of  plant  there  exists  a  considerable 
variety  of  organic  acids.3 

But  the  effects  of  plant  growth  are  not  necessarily  always 
destructive  ;  such  may  be  conservative  or  even  protective.  In 
glaciated  regions,  it  is  often  the  case  that  the  striated  and  pol- 
ished surfaces  of  the  rocks  have  been  preserved  only  where  pro- 
tected from  the  disintegrating  action  of  the  sun  and  atmosphere 

1  It  is  stated  by  Storer  (Chemistry  as  applied  to  Agriculture)   that  some 
lichens  have  been  found  to  contain  half  their  weight  of  oxalate  of  lime. 

2  Aughey  has  found  roots  of  the  buffalo  berry  (Sherperdia  argophylla')  pene- 
trating the  loess  soils  of  Nebraska  to  the  depth  of  50  feet. 

3  See  Application  of  Organic  Acids  to  the  Examination  of  Minerals,  H.  Car- 
rington Bolton,  Proc.  Am.  Assoc.  for  the  Advancement  of  Science,  XXXI,  1883, 
and  Available  Mineral  Plant  Food  in  Soils,  B.  Dyer,  Jour.  Chem.  Society,  March, 
1894. 


ACTION   OF  PLANTS   AND   ANIMALS  203 

by  a  thin  layer  of  turf  or  moss.  As  a  general  rule,  however, 
the  manifest  action  of  plant  growth  is  to  accelerate  chemical 
decomposition,  through  keeping  the  surfaces  continually  moist, 
and  to  retard  erosion.  (See  further  on  p.  280.) 

Action  of  Bacteria.  —  The  researches  of  A.  Miintz,1  Wido 
gradsky,  Schlosing,  and  others  tend  to  show  that  bacteria  may 
exercise  a  very  important  influence  in  promoting  rock  disinte- 
gration and  decomposition.  Their  influence  in  promoting  nitri- 
fication has  been  already  alluded  to.  It  would  appear  that 
while  these  organisms  secrete  and  utilize  for  their  sustenance 
the  carbon  from  the  carbonic  acid  of  the  atmosphere,  as  do 
plants  of  a  higher  order,  they  may  also  assimilate  carbonate 
of  ammonium,  forming  from  it  organic  matter  and  setting  free 
nitric  acid.  Being  of  microscopic  proportions,  the  organisms 
penetrate  into  every  little  cleft  or  crevice  produced  by  atmos- 
pheric agencies,  and  throughout  long  periods  of  time  produce 
results  of  no  inconsiderable  geological  significance.  The  depth 
below  the  surface  at  which  such  may  thrive  is  presumably  but 
slight,  and  their  period  of  activity  limited  to  the  summer  months. 
They  have  been  found  on  rocks  of  widely  different  character  — 
granites,  gneisses,  schists,  limestones,  sandstones,  and  volcanic 
rocks  —  and  on  high  mountain  peaks  as  well  as  on  lower  levels. 
The  Pic  Pourri,  or  Rotten  Peak,  in  the  Lower  Pyrenees  of  south- 
western France,  is  composed  of  friable  and  superficially  decom- 
posed calcareous  schists,  throughout  the  whole  mass  of  which 
are  found  the  nitrifying  bacteria,  which  are  believed  to  have 
been  instrumental  in  promoting  its  characteristic  decomposition. 
The  organism  acts  even  upon  the  most  minute  fragments,  reduc- 
ing them  continually  to  smaller  and  smaller  sizes.  Each  frag- 
ment loosened  from  the  parent  mass  is  found  coated  with  a  film 
of  organic  matter  thus  produced,  and  the  accumulation  begun 
by  these  apparently  insignificant  forces  is  added  to  by  residues 
of  plants  of  a  higher  order,  which  come  in  as  soon  as  food  and 
foothold  are  provided.2 

Mr.   J.   E.   Mills,3  and  after  him  J.   C.   Branner,4  lay  con- 
siderable stress  on  the  decomposing  effect  of  vegetable  matter 

1  Comptes  Kendus  de  1' Academic  des  Sciences,  CX,  1890,  p.  1370. 

2  It  is,  perhaps,  as  yet,  too  early  to  say  to  what  extent  the  presence  of  bacteria 
may  be  incidental  to  decomposition,  rather  than  causative. 

3  American  Geologist,  June,  1889,  p.  357. 

4  Bull.  Am.  Geol.  Soc.  of  America,  Vol.  VII. 


204        THE   PRINCIPLES   INVOLVED   IN   ROCK- WEATHERING 

carried  into  the  ground  by  ants  in  certain  parts  of  Brazil,  Mills 
going  so  far  as  to  describe  the  ants  as  continually  pouring  car- 
bonic acid  into  the  ground.  Be  this  as  it  may,  the  evacuations 
of  the  ants  themselves  are  undoubtedly  of  such  a  nature  as  to 
further  the  processes  of  decomposition.  Certain  species  of  ants, 
locally  known  as  saubas,  or  sauvas,  live,  according  to  Branner, 
in  enormous  colonies,  burrowing  in  the  earth,  where  they  exca- 
vate chambers  with  galleries  that  radiate  and  anastomose  in 
every  direction,  and  into  which  they  carry  great  quantities  of 
leaves.  Certain  species  of  termites,  the  white  ants  of  Brazil,  are 
also  active  promoters  in  bringing  about  changes  in  the  structure 
of  the  soil,  and  incidentally  accelerating  decomposition.  The 
organic  matter  carried  by  these  creatures  into  the  ground,  there 
to  decompose,  furnishes  organic  acids  to  promote  further  decay 
in  the  material  close  at  hand,  and  by  its  downward  percolation 
to  attack  the  still  firm  rocks  at  greater  depths.  Indeed,  these 
numerous  channels,  through  affording  easy  access  of  air  and 
surface  waters  with  all  their  absorbed  gases  or  alkaline  salts, 
may  serve  indirectly  a  geological  purpose  scarcely  inferior  to 
that  of  the  joints  in  massive  rocks.  (See  further  under  soil 
modified  by  plant  and  animal  life.) 

The  mechanical  agency  which  has  already  been  referred 
to  as  instrumental  in  bringing  about  a  certain  amount  of  de- 
composition in  silicate  minerals,  is  greatly  augmented  when 
such  trituration  takes  place  in  connection  with  organic  matter. 
J.  Y.  Buchanan  has  shown,1  that  the  mud  of  sea-bottoms  is  being 
continually  passed  and  repassed  through  the  alimentary  canals 
of  marine  animals,  and  that  in  so  doing  the  mineral  matter  not 
merely  undergoes  a  slight  amount  of  comminution  and  conse- 
quent decomposition,  but  a  chemical  reduction  takes  place 
whereby  existing  sulphates  are  converted  into  sulphides.  Such 
sulphides  and  the  metallic  constituents  of  the  silicates  and  other 
compounds,  particularly  those  of  iron  and  manganese,  would 
on  exposure  to  sea-water  become  converted  into  oxides.  It  is 
through  such  agencies  that  he  would  account  for  the  presence 
of  sulphur  in  marine  muds,  and  the  variations  in  color,  from 
shades  of  red  or  brown  to  blue  and  gray,  in  the  former  the  iron 
occurring  as  oxides,  while  in  the  latter  it  exists  as  a  sulphide. 
Of  course  either  form  may  be  more  or  less  permanent  according 

1  On  the  Occurrence  of  Sulphur  in  Marine  Muds,  Proc.  Royal  Soc.  of  Edin- 
burgh, 1890-91. 


THE    PRODUCTION   OF   CARBONATES  205 

as  the  mud  may  be  devoid  of  animal  life,  or  protected  from 
oxidizing  influences.  These  reactions,  being  subaqueous,  are 
somewhat  beyond  the  scope  of  the  present  work,  but  are  never- 
theless not  without  interest  in  this  connection. 

One  of  the  most  conspicuous  results  of  rock-weathering 
through  the  agencies  of  water  and  organic  acids,  as  above  enu- 
merated, is  manifested  in  the  production  of  carbonates  of  lime 
and  more  rarely  of  magnesia,  iron,  and  the  alkalies.  Thus  in 
the  decomposition  of  lime-bearing  silicates,  as  the  feldspars, 
pyroxenes,  and  amphiboles,  the  lime  almost  invariably  separates 
out  as  calcite  or  aragonite,  and  often  may  be  found  filling  cracks 
and  crevices,  as  veins  of  "  spar  "  in  the  very  rock  masses  from 
which  it  was  derived.  The  celebrated  verde  di  Genova  and 
vercle  di  Prato  marbles  are  but  secondary  rocks  derived  by 
hydration  from  pre-existing  pyroxenic  masses  and  in  which  the 
lime  and  magnesia  have  separated  out  as  carbonates  forming 
the  white  veins  by  which  the  stone  is  traversed.  The  almost 
universality  of  carbonate  formation  incident  to  rock-weathering 
manifests  itself  in  the  ready  effervescence  of  freshly  decomposed 
material  when  treated  with  an  acid.  It  is  indeed  difficult  to 
find  weathered  rocks  of  any  kind  that  will  not  show  at  least 
traces  of  secondary  carbonates,  of  which  those  of  calcium  are  by 
far  the  more  abundant. 

It  is  further  to  be  noted  that  the  solvent  and  general  chemical 
activity  of  water  is  often  greatly  augmented  by  the  salts  and 
acids  it  acquires  through  the  decomposition  of  various  minerals 
with  which  it  comes  in  contact.  Thus  through  the  decomposi- 
tion of  iron  pyrites  there  may  be  formed  free  sulphuric  acid, 
or  through  the  decomposition  of  a  feldspar,  carbonates  of  the 
alkalies,  any  of  which,  when  in  solution,  are  more  energetic 
factors  in  promoting  decomposition  than  water  alone.  Hence 
under  certain  conditions  the  process  of  decomposition  once  set 
in  operation  augments  itself,  and  goes  on  with  increasing  vigor 
until  such  a  depth  is  reached  that  the  percolating  solutions 
become  neutralized  and  further  action,  aside  from  hydration, 
practically  ceases. 


THE  WEATHERING  OF  BOCKS  (Continued} 
II.    CONSIDERATION   OF   SPECIAL   CASES 

Let  us  now  enter  into  a  consideration  of  the  composition  of 
a  few  prominent  rock  types,  and  note  the  changes  they  have 
undergone  in  this  process  of  weathering,  assuming,  as  we  must 
for  the  time  being,  that  they  have  been  all  subjected  to  essen- 
tially the  same  conditions.  Inasmuch,  as  has  been  noted  already, 
there  are  divers  types  of  rocks,  differing  not  merely  in  chemical 
and  mineral  composition,  but  in  structure  as  well,  it  is  an  easy 
assumption  that  the  results  of  prolonged  weathering  may  be 
widely  divergent.  Yet,  as  will  become  apparent,  the  ultimate 
products  from  all  but  the  purely  quartzose  rocks,  present  strik- 
ing similarities. 

In  the  tables  following  are  given  the  results  of  chemical  and 
mechanical  analyses  of  rocks  of  various  kinds  and  in  varying 
stages  of  degeneration.  We  will  begin  with  a  consideration 
of  the  granitic  rocks  of  the  District  of  Columbia.1 

The  rock  (see  PL  14)  in  its  fresh  condition  is  a  strongly 
foliated  gray  micaceous  granite  showing  to  the  unaided  eye 
a  finely  granular  aggregate  of  quartz  and  feldspars  arranged  in 
imperfect  lenticular  masses  from  2  to  5  mm.  in  diameter,  about 
and  through  which  are  distributed  abundant  folia  of  black 
mica.  In  the  thin  section  the  structure  is  seen  to  be  cataclastic. 
Quartz  and  black  mica  are  the  most  prominent  constituents, 
though  there  are  abundant  feldspars  of  both  potash  and  soda- 
lime  varieties,  "which,  owing  to  their  limpidity,  can  by  the 
unaided  eye  scarcely  be  distinguished  from  the  quartz.  The 
potash  feldspar  has  in  part  a  microcline  structure.  Aside  from 
these  minerals,  a  primary  epidote,  in  small  granules  and  at  times 
quite  perfectly  outlined  crystals,  is  a  strikingly  abundant  con- 
stituent. Small  apatites,  a  few  flakes  of  white  mica  (sericite), 

1  Disintegration  of  the  Granitic  Rocks  of  the  District  of  Columbia,  Bull.  Geol. 
Soc.  of  America,  Vol.  VI,  1895,  pp.  321,  332. 

206 


WEATHERING   OF   GRANITE 


207 


and  widely  scattering  black  tourmalines  and  iron  ores  complete 
the  list  of  recognizable  minerals. 

The  outcrops  from  which  the  samples  for  the  analyses  to 
which  attention  is  first  called  were  selected  are  shown  in  the 
plate.  At  the  very  bottom,  the  rock  is  hard,  fresh,  and  com- 
pact, without  trace  of  the  decomposition  products  other  than 
as  indicated  by  minute  infiltrations  of  calcite  from  above.  Just 
above  the  level  of  the  small  creek  which  flows  at  the  foot  of 
the  bluff,  at  the  point  indicated  by  the  first  series  of  right-and- 
left  joints  near  the  centre  of  the  view,  the  character  of  the  rock 
changes  quite  suddenly,,  becoming  brown  and  friable,  though 
still  retaining  its  form  and  easily  recognizable  granitic  appear- 
ance. A  few  feet  above  a  third  zone  begins,  in  which  the  rock 
is  converted  into  sand  and  gravel  and  which  becomes  more  and 
more  soil-like  to  the  top  of  the  bank,  where  it  becomes  admixed 
with  organic  matter  from  the  growing  plants.  The  amount 
of  organic  matter  is  quite  small,  however,  and  in  making  the 
analyses  care  was  taken  to  remove  such  as  was  recognizable  in 
the  form  of  rootlets,  leaves,  and  twigs. 

Bulk  analyses  of  these  three  types,  (I)  fresh  gray  granite, 
(II)  brown  but  still  moderately  firm  and  intact  rock,  and  (III) 
the  residual  sand,  yielded  the  results  given  in  the  columns  cor- 
respondingly numbered  below:  — 


CONSTITUENTS 

I 

II 

in 

Ignition  .... 

1.22% 

3.27  % 

4.70% 

Silica  (Si02)     

09.33 

66.82 

65.69 

Titanium  (Ti02)  
Alumina  (A1203)  
Iron  protoxide  (FeO) 

not  det. 
14.33 
3.G01 

not  det. 
15.62 
1.69 

0.31 
15.23 

Iron  sesquioxide  (Fe203)  
Lime  (CaO)      

3.21 

1.88 
3.13 

4.39 
2.63 

Magnesia  (MgO)  

2.44 

2.76 

2.64 

Soda  (Na20) 

2  70 

2.58 

2.12 

Potash  (K20) 

2.67 

2.04 

2.00 

Phosphoric  acid  (P2C>5)     

0.10 

not  det. 

0.06 

99.60  % 

99.79% 

99.77  % 

In  glancing  over  these  figures  it  is  at  once  apparent  that 
there  is  a  surprisingly  small  difference  in  ultimate  composition 

1  4.00%  when  calculated  as  Fe203. 


208  ROCK   DISINTEGRATION   AND   DECOMPOSITION 

between  the  sound  rock  and  the  residual  sand,  the  more  marked 
differences  being  a  slightly  smaller  amount  of  silica,  more  alu- 
mina, and  slightly  diminished  amounts  of  lime,  magnesia,  pot- 
ash, and  soda,  with  a  considerable  increase  in  the  amount  of 
water.  The  ferrous  salts  have  moreover  been  converted  into 
ferric  forms.  It  does  not  necessarily  follow,  however,  that  no 
more  actual  gain  or  loss  of  material  or  change  in  manner  of 
combination  than  is  here  indicated  may  not  have  taken  place, 
and  at  the  very  outset  it  may  be  well  to  enter  into  a  discussion 
of  the  manner  in  which  the  results  of  such  analyses  are  to  be 
considered. 

We  must  first  of  all  remember  that  any  indicated  loss  or 
gain  of  a  constituent  may  be  only  apparent,  and  that  the  true 
relative  proportions  can  be  learned  only  by  calculating  results 
of  analyses  of  both  fresh  and  decomposed  materials  on  a  com- 
mon basis.  Thus  the  first  glance  at  analysis  III,  as  given, 
might  lead  one  to  surmise  that  the  decomposed  rock  had  actually 
lost  only  some  3.3%  of  silica.  This,  however,  is  not  strictly 
the  case,  since  this  analysis  shows  4.7%  volatile  constituents 
against  1.22%  in  analysis  I  of  the  fresh  material.  Could  we 
assume  that  this  difference  of  3.48  %  was  due  wholly  to  a 
uniform  absorption  of  moisture,  as  by  a  clay,  the  problem  would 
resolve  itself  into  simply  recalculating  all  analyses  upon  a 
water-free  basis. 

The  results  obtained  thus  are  not  quite  satisfactory,  however, 
and  it  is  thought  a  more  correct  view  of  the  changes  taking 
place  may  be  obtained  by  assuming  for  one  of  the  constituents 
a  fairly  constant  value  and  using  this  as  a  basis  for  comparison. 

Of  all  the  essential  constituents  occurring  in  appreciable 
quantities  in  siliceous  crystalline  rocks  the  alumina  and  the  iron 
oxides  are  the  most  refractory  and  the  least  liable  to  be  removed 
by  a  leaching  process,  although  they  may  undergo  manifold 
changes  in  mode  of  combination.  Although  not  absolutely 
correct,  therefore,  we  will  for  our  present  purposes  assume  the 
one  or  the  other  of  these  (in  this  case  the  iron  as  Fe2O3)  as  a 
constant  factor,  and  in  order  to  show  the  proportional  or  actual 
amount  of  loss  of  any  constituent  will  recalculate  the  analyses 
upon  this  basis,  a  proceeding  for  which,  so  far  as  alumina  is 
concerned,  we  have  already  good  authority.1  This  method  will 
be  adopted,  however,  only  with  the  siliceous  crystalline  rocks, 
1  G.  Roth,  Allegemeine  u.  Chemische  Geologic,  3d  ed. 


WEATHERING   OF  GRANITE 


209 


in  which,  for  reasons  noted  later,  the  process  of  decomposition, 
we  have  reason  to  suppose,  is  more  complex  than  in  calcareous 
and  magnesian  rocks  poor  or  lacking  in  the  alkalies.  The 
entire  discussion  is  one  beset  with  great  difficulties,  since  we 
lack  definite  knowledge  as  to  the  exact  processes  which  have 
been  going  on  and  need  constantly  to  guard  against  assump- 
tions too  hastily  drawn  or  based  upon  insufficient  data.  Indeed, 
any  assumption  based  upon  the  results  of  chemical  analyses 
alone  is  likely  to  lead  to  grave  error. 

If,  then,  we  consider  the  iron  in  the  form  of  Fe2O3  as  a  constant 
factor,  we  may,  by  proper  calculation,  obtain  the  results  given 
in  column  (IV)  below,  which  represent  the  proportional  gain 
and  loss  of  the  various  constituents  of  the  rock  in  passing  from 
the  condition  indicated  in  column  (I)  above,  to  that  indicated 
in  column  (III).  Such  a  comparison  is  instructive  as  showing 
not  merely  the  relative  loss  and  gain,  but  also  the  total  loss  of 
material,  in  this  case  13.47  %,  accompanied  by  a  gain  of  2.16%, 
in  volatile  matter. 


DISINTEGRATED  AND  DECOMPOSED  GRANITE,  DISTRICT  OF  COLUMBIA,  SHOWING 
PROPORTIONAL  Loss  OF  CONSTITUENTS 


• 

IV 

V 

VI 

CONSTITUENTS 

PERCENTAGE 
Loss  FOR  EN- 
TIKE  ROCK 

PERCENTAGE 
OF  EACH  CON- 
STITUENT SAVED 

PERCENTAGE 
OF  EACH  CON- 
STITUENT LOST 

Silica  (Si02)     
Alumina  (Al20s) 

10.50% 
0.46 

85.11% 
96.77 

14.89% 
3.23 

Iron  sesquioxide  (Fe203)  
Iron  protoxide  (FeO) 

|        0.00 

100.00 

0.00 

Lime  (CaO)     

0.81 

74.79 

25.21 

Magnesia  (MgO)  
Soda  (Na20) 

0.36 
0.77 

98.51 
71.38 

1.49 

28.62 

Potash  (K20) 

0.85 

68.02 

31.98 

Phosphoric  anhydride  (P205)     .     .     . 
Ignition   .                             .... 

0.04 
2.161 

60.00 
100.00 

40.00 
0.00 

Total  loss 

13.47  % 

Such  results  are  still  far  from  satisfactory,  and  it  is  believed 
the  tables  will  be  more  useful  and  instructive  can  we  show  the 


Gain. 


210  ROCK   DISINTEGRATION  AND   DECOMPOSITION 

percentage  loss  and  gain  of  each  constituent  as  compared  with 
the  same  constituent  in  the  original  rock.  This  can  also  readily 
be  accomplished  by  a  process  the  formula  for  which  is  given 
below,1  and  by  which  are  obtained  the  results  given  in  columns 
V  and  VI. 

From  a  perusal  of  these  figures,  it  appears  that  the  residual 
sand  retains  85.11%  of  the  original  silica;  96.77%  of  the  alumina; 
all  the  ferric  oxide;  74.79%  of  its  lime;  98.51%  of  its  mag- 
nesia, together  with  71.38  %  of  its  soda  and  68.02  of  the  potash, 
while  there  has  been  an  actual  gain,  as  was  to  be  expected,  in 
volatile  matter. 

Let  us  not,  however,  too  hastily  assume  that  we  have  ex- 
hausted the  subject. 

We  must  remember,  further,  that  while  an  analysis  shows  the 
actual  composition  of  a  rock  so  far  as  the  various  elements  are 
concerned,  it  quite  fails  to  show  the  manner  in  which  those 
elements  are  combined.  While  the  ultimate  composition  of 
the  fresh  and  decomposed  samples  may  be  closely  similar,  it  is 
possible,  indeed  probable,  that  in  some  cases  at  least  the  manner 
of  combination  of  these  elements  is  quite  different.  This  is 
well  illustrated  in  the  case  of  the  figures  showing  the  percent- 
ages of  alumina  in  analyses  I  and  III  and  which  differ  only 
nine-tenths  of  one  per  cent  in  total  amount;  yet  in  the  first  the 
alumina  exists  mainly  in  the  form  of  anhydrous  silicates  of 
alumina,  potash,  iron,  and  magnesia  (as  in  the  feldspars  and 
mica),  while  in  the  last  a  very  considerable  proportion,  or  indeed 
all  in  extreme  cases  of  weathering,  may  exist  as  a  hydrous  sili- 
cate of  alumina  only  (kaolin).  It  is  in  instances  of  this  kind 
that  the  microscope  may  render  efficient  service,  and  much  may 
be  learned  by  means  of  such  mechanical  analyses  as  can  be  made 
by  sifting  and  washing.  Such  separations  made  on  this  disin- 
tegrated rock  showed  it  to  consist  of  particles  as  given  in  the 
following  table,  the  4.25%  silt  being  obtained  by  washing  the 

1  The  formula  employed  in  these  calculations  is  as  follows :  — — -  =  x  :  and 

Jj  X  O 

100  —  x  —  y,  in  which  A  =  the  percentage  of  any  constituent  in  the  residual 
material ;  B  =  the  percentage  of  the  same  constituent  in  the  fresh  rock,  and 
C  =  the  quotient  obtained  by  dividing  the  percentage  amount  of  alumina  (or 
iron  sesquioxide,  whichever  is  taken  as  a  constant  factor)  of  the  residual  mate- 
rial by  that  in  the  fresh  rock,  the  final  quotient  being  multiplied  by  100.  x  then 
equals  the  percentage  of  the  original  constituent  saved,  in  the  residue,  and  y  the 
percentage  of  the  same  constituent  lost. 


WEATHERING  OF   GRANITE  211 

10.75%  of  material  which  passed  through  fine  bolting-cloth  of 
120  meshes  to  the  lineal  inch,  and  which  represents  the  impal- 
pable mud  remaining  in  suspension  while  the  6.5  %  of  fine  sand 
sank  quickly  to  the  bottom  of  the  beaker  in  which  the  washing 
was  made.  The  residual  sand  yielded  then:  - 

Silt 4.25%      Largest  grains  0. 1    mm.  in  diameter 

Very  fine  sand 6.50  "  "  0.18 

Fine  sand 11.25  "  "  0.25 

Medium  sand 3.80  "  »  0.65 

Sand) 11.00  "  "  1.00 

Sand/ 23.50  "  "  1.50 

Coarse  sand 29.50  "  "  2.00 

Gravel    .  10.20  "  »  8.00 


Total 100.00% 

The  coarser  of  these  particles,  like  the  gravel  and  coarse 
sand,  are  of  a  compound  nature,  being  aggregates  of  quartz 
and  feldspar,  with  small  amounts  of  mica  and  other  minerals. 
In  the  finer  material,  on  the  other  hand,  each  particle  repre- 
sents but  a  single  mineral,  the  process  of  disaggregation  having 
quite  freed  it  from  its  associates,  excepting,  of  course,  the 
microscopic  inclusions  which  could  be  liberated  only  by  a 
complete  disintegration  of  the  host  itself.  These  particles, 
as  seen  under  the  microscope,  are  all  sharply  angular,  and  in 
many  cases  surprisingly  fresh,  though  the  analyses,  as  given 
above,  had  suggested  only  a  slight  change  in  chemical  composi- 
tion. The  mica  shows  the  greatest  amount  of  alteration,  the 
change  consisting  mainly  in  an  oxidation  of  its  ferruginous 
constituent,  whereby  the  folia  becomes  stained  and  reduced 
to  yellowish  brown  shreds.  The  feldspars  are,  in  some  cases, 
opaque  through  kaolinization,  but  in  others  are  still  fresh  and 
unchanged  even  in  the  smallest  particles.  The  finest  silt, 
when  treated  with  a  diluted  acid  to  remove  the  iron  stains, 
shows  the  remaining  granules  of  quartz,  feldspar,  and  epidote 
beautifully  fresh,  and  with  sharp,  angular  borders,  the  mica 
being,  however,  almost  completely  decolorized. 

An  analysis  of  the  silt,  which  was  found  to  constitute  4.25% 
of  the  entire  mass  of  disintegrated  material,  as  noted  above, 
is  given  below,  and  also  a  partial  separation  and  analysis  of 
the  39.7%  soluble,  and  60.3%  insoluble  portions.1 

1  In  all  analyses  made  by  or  under  the  direction  of  the  author,  the  matter 
tabulated  as  soluble  is  that  extracted  by  boiling  for  three  hours  in  hydrochloric 


212  ROCK  DISINTEGRATION  AND   DECOMPOSITION 

ANALYSES  OP  SILT  PROM  DISINTEGRATED  GRANITE 


CONSTITUENTS 

I 

II 

III 

BULK  ANALYSIS 

OF  SILT 

ANALYSIS  OF 
SOLUBLE  PORTION 
(39.7%)  SILT 

ANALYSIS  OF 
INSOLUBLE  PORTION 
(60.3  %)  SILT 

Ignition     

8.12% 
49.39        { 

23.84 
3.69 
4.41         } 
4.60        -1 
3.36         [ 
2.49        J 

8.12% 
InHCl        1.123 
InNa2C0311.147 
9.-21 
4.47 

Not  det. 

0.97% 

}       37.30 

13.40 
0.82 

r            2.90 

J       Trace 
j          2.75 
1.07 

Silica  (Si02)      

Alumina  (A1203)   .... 
Iron  sesquioxide  (Fe20s)    . 
Lime  (CaO)       .    .         .     . 

Magnesia  (MgO)    .... 
Soda  (Na20) 

Potash  (K20)         .... 

99.90% 

34.07 

59.21 

93.28  % 

From  these  analyses  it  would  appear  that  of  the  17  grammes 
of  silt,  representing  4%  of  the  total  disintegrated  material, 
only  39.7%  is  soluble  ;  and,  further,  that  a  very  considerable 
proportion  of  the  insoluble  residue,  as  indicated  by  the  high 
percentages  of  alkalies  and  lime,  still  consist  of  unaltered  soda- 
lime  and  potash  feldspars,  the  iron  and  magnesia  alone  having 
been  largely  removed. 

These  results  are  not  quite  what  one  would  be  led  to  expect 
from  a  perusal  of  the  literature  bearing  upon  the  subject  of 
rock  decomposition.  As  long  since  noted  by  J.  G.  Forch- 
hammer,  G.  Bischof,  T.  Sterry  Hunt,  and  others,  the  ordinary 
processes  of  decay  in  siliceous  rocks  containing  ferruginous 
protoxides  and  alkalies  consists  in  the  higher  oxidation  and 
separation  of  the  protoxides  in  the  form  of  hydrous  sesqui- 
oxides  and  a  general  hydration  of  the  alkaline  silicates,  accom- 
panied by  the  formation  of  alkaline  carbonates,  which,  being 
readily  soluble,  are  taken  away  nearly  as  fast  as  formed.  More 
or  less  silica  is  also  removed,  according  to  the  amount  of  car- 
bonic acid  present,  a  portion  of  the  alkalies  forming  soluble 


acid  of  one-half  normal  strength,  to  which  is  added  the  silica  set  free  in  a  gelati- 
nous form  by  the  acid  and  subsequently  extracted  by  sodium  carbonate  solu- 
tion. All  analyses  made  on  material  first  dried  at  100°  C. 


WEATHERING   OF   GRANITE  213 

alkaline  silicates  when  the  supply  of  the  acid  is  insufficient  to 
take  them  all  up  in  the  form  of  carbonates.  The  apparent 
anomaly  here  shown  is  partially  explained  by  examination  of 
the  various  separations  with  the  microscope.  Thus  the  low 
percentage  of  silica  is  found  to  be  in  large  part  due  to  the  fact 
that  the  residual  quartz  granules  are,  in  many  cases,  too  large 
to  pass  the  120-mesh  sieve,  or,  if  passing,  have  been  largely 
separated  in  the  process  of  washing.  Further,  it  is  found  that 
the  sifting  has  served  to  concentrate  the  small  epidotes  in  the 
fine  sand,  and  a  portion  of  them  have  even  come  over  with 
the  silt.  The  presence  of  this  epidote  also  explains  in  part  the 
high  percentage  of  lime  shown,  since  the  mineral  itself  carries 
some  20  to  24  %  of  this  material.  The  large  percentages  of 
magnesia,  soda,  and  potash  cannot,  however,  be  thus  accounted 
for,  and  we  are  led  to  infer  that  either  these  elements  are  there 
combined  in  minute  amorphous  zeolitic  compounds,  unrecog- 
nizable as  such  under  the  microscope,  or,  as  seems  more  prob- 
able, the  feldspathic  constituents,  to  which  the  alkalies  are  to  be 
originally  referred,  have  undergone  a  mechanical  splitting  up 
rather  than  a  chemical  decomposition.  This  view  is,  to  a 
certain  extent,  borne  out  by  microscopic  studies,  but  it  is  diffi- 
cult to  measure  by  the  eye  the  relative  abundance  of  these 
constituents  with  sufficient  accuracy  to  enable  one  to  form  any 
satisfactory  conclusion.  The  magnesia  must  come  from  the 
shreds  of  mica,  many  of  which,  from  their  small  size  and  almost 
flocculent  nature  when  decomposed,  would  naturally  be  found 
in  the  silt  obtained  as  stated. 

It  is  to  be  noted  that  the  magnesia,  together  with  the  iron, 
exists  almost  wholly  in  a  soluble  form. 

It  is  evident  at  once  that  we  have  had  to  do  here  with 
but  the  preliminary  stages  of  granitic  weathering,  that  the 
process  is  more  one  of  disintegration  than  decomposition,  and 
it  will  be  well  to  consider  now  a  case  in  which  the  decom- 
position has  gone  on  to  the  condition  of  a  residual  clay,  as 
found  in  many  of  the  Southern  states.  For  this  purpose  a 
biotite  gneiss  or  gneissoid  granite  found  near  North  Garden, 
in  Albemarle  County,  Virginia,  is  selected.  The  rock  is  a 
coarse  gray  feldspar-rich  variety  with  abundant  folia  of  black 
mica.  Under  the  microscope  it  shows  the  presence  of  both 
potash  and  soda-lime  feldspars,  a  sprinkling  of  apatite  and 
iron  ores,  sporadic  occurrences  of  an  undetermined  zeolite,  and 


214  ROCK   DISINTEGRATION  AND   DECOMPOSITION 

an  extraordinary  number  of  minute  zircons  which  are  mostly 
enclosed  in  the  feldspars.  There  are  also  present  occasional 
small  garnets  and  aggregates  of  decomposition  products  the 
exact  nature  of  which  was  not  made  out.  The  residual  soil 
resulting  from  the  decomposition  of  this  rock  is  highly  plastic, 
of  a  deep  red-brown  color,  and  has  a  distinct  gritty  feeling  in 
the  hand,  owing  to  the  presence  of  quartz  and  undecomposed 
silicate  minerals.  In  columns  I  and  III  below  are  given  the 
results  of  analyses  of  fresh  rock  and  residual  soil,  and  in  II,  IV, 
and  V  the  analyses  of  the  soluble  and  insoluble  portions.  In 
columns  VI,  VII,  and  VIII  are  given  the  calculated  percentage 
amounts  of  the  various  constituents  saved  and  lost,  as  before. 

The  particular  features  to  which  attention  need  here  be 
called,  are  (1)  that  30.47  %  of  the  fresh  rock  and  69.18  %  of 
the  decomposed  are  soluble  in  hydrochloric  acid  and  sodium 
carbonate  solutions,  and  that  more  than  half  the  potash  and 
nearly  the  same  proportion  of  the  soda  in  the  fresh  rock  is 
found  in  the  acid  extract.  (2)  That  the  insoluble  portion  of 
the  residuary  material  is  mainly  in  the  form  of  free  quartz. 
(3)  That  44.67  %  of  the  original  matter  has  been  leached  away, 
and  that  (4)  of  the  original  silica  52.45  %  is  lost,  while  85.61  % 
of  the  iron  and  all  the  alumina  remain.  All  the  lime  has  dis- 
appeared, 83.52  %  of  the  potash,  95.03  %  of  the  soda,  and  74.70  % 
of  the  magnesia.  The  total  amount  of  water,  as  indicated  by 
the  ignition,  has  increased  very  greatly,  as  was  to  be  expected. 
The  small  original  amount  of  phosphoric  acid  prohibits  our 
placing  too  much  reliance  upon  the  indicated  gain  in  this  con- 
stituent, since  it  may  be  due  to  errors  in  manipulation. 

Passing  from  the  acid  group  of  granular  crystalline  rocks, 
we  will  consider  next  a  closely  allied  form  differing  mainly  in 
the  absence  of  quartz  as  an  essential  constituent,  and  in  the 
presence  of  eleeolite,  the  elseolite  syenites  of  the  Fourche  Moun- 
tain region  of  Arkansas.  These  are  somewhat  coarsely  crystal- 
line granitic-appearing  rocks,  in  which  an  orthoclase  feldspar 
in  broadly  tabular  forms  is  the  prevailing  constituent,  though 
always  accompanied  by  nepheline,  biotite,  pyroxene,  titanite, 
and  apatite,  while  fluorite,  analcite,  and  thomsonite,  together 
with  calcite,  occur  as  secondary  products.  The  rock  weathers 
away  to  a  coarse  gray  gravel  which  ultimately  becomes  a  clay, 
from  which,  by  washing,  may  be  obtained  kaolin  in  a  fail- 
degree  of  purity. 


WEATHERING   OF   GNEISS 


215 


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216 


KOCK  DISINTEGRATION  AND   DECOMPOSITION 


The  following  analyses  from  the  work  of  Dr.  J.  F.  Williams1 
will  serve  to  show  the  changes  which  have  here  taken  place  in 
the  transformation  from  (I)  fresh  syenite  through  (II  and  III) 
intermediate  stages  of  decomposition  to  (IV)  a  kaolin-like 
residue. 

ANALYSES  OF  FRESH  AND  DECOMPOSED  SYENITE,  ARKANSAS 


CONSTITUENTS 

I 

II 

III 

IV 

Silica  (Si02)    .... 

59.70% 

58.50% 

50.65% 

46.27  % 

Alumina  (Al20s)      .     . 

18.85 

25.71 

26.71 

38.57 

Ferric  oxide  (Fe203)     . 

4.85 

3.74 

4.87 

1.36 

Lime  (CaO)     .... 

1.34 

0.44 

0.62 

0.34 

Magnesia  (MgO)  .     .     . 

0.68 

Trace 

0.21 

0.25 

Potash  (K20)  .... 

5.97 

1.96 

1.91 

0.23 

Soda  (Na20)    .... 

6.29 

1.37 

0.62 

0.37 

Ignition  (H20)     .     .     . 

1.88 

5.85 

8.68 

13.61 

99.56  % 

97.57  % 

94.27  % 

101.00% 

Recalculating  the  numbers  given  in  columns  I  and  IV  upon 
the  basis  of  100,  we  may  obtain  by  further  calculation,  as  already 
described,  the  figures  given  in  columns  V  and  VI  and  VII  below, 
which  represent  the  proportional  loss  of  each  constituent,  as 
before. 


CALCULATED  Loss  OF  MATERIAL 


v 

VI 

VII 

CONSTITUENTS 

PERCENTAGE 
Loss  FOR  ENTIRE 
KOCK 

PERCENTAGE 
OF  EACH  CON- 
STITUENT SAVED 

PERCENTAGE 
OF  EACH  CON- 
STITUENT LOST 

Silica  (Si02)    . 

37.  28%  loss 

37.82% 

62.18% 

Alumina  (Al20a) 

0.00 

100.00 

0.00 

Ferric  oxide  (Fe203)    

4.19 

13.83 

86.17 

Lime  (CaO) 

1  19 

12.10 

87.90 

Magnesia  (MgO) 

0.57 

17.90 

82.10 

Potash  (K20)  

5.90 

18.15 

81.85 

Soda  (Na20)  

6.15 

2.89 

97.11 

Water  (H20)  .... 

0  00 

100.00 

0.00 

Total  loss  of  original  material,  56.28%. 
1  Ann.  Eep.,  Vol.  II,  1890,  Arkansas  Geol.  Survey. 


WEATHERING   OF    SYENITE    AND    PHONOLITE 


217 


Here,  as  with  the  granitic  rocks,  it  will  be  noted  we  have  a 
gradual  increase  in  the  percentage  of  water  as  the  decomposi- 
tion advances,  and  a  decrease  in  the  amount  of  silica  even  more 
pronounced.  This  last,  as  may  be  readily  imagined,  is  due  to 
the  absence  of  free  quartz  in  the  Fourche  Mountain  rocks. 

The  phonolites  of  Marienfels,  near  Assig,  in  Bohemia,  have 
been  described  by  Lemberg  l  as  weathering  into  a  bright- 
colored,  porous,  friable  mass,  the  composition  of  which,  as 
compared  with  the  fresh  rock,  is  shown  below.  Each  column, 
it  should  be  stated,  represents  an  average  of  three  analyses, 
I  being  the  fresh  and  II  the  weathered  material,  while  in  III, 
IV,  and  V  are  given  the  percentage  calculations  of  gain  and 
loss,  as  before. 

ANALYSES  OF  FRESH  AND  DECOMPOSED  PHONOLITE,  BOHEMIA 


I 

II 

ill 

IV 

V 

CONSTITUENTS 

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Silica  (8162) 

55.67  % 

55.  72  % 

4.83  % 

91  46  % 

8  54  % 

Alumina  (A^Os)    ...          . 

20.64 

22.19 

0.37 

98.40 

1.60 

Ferric  oxide  (Fe203)  
Lime  (CaO)        
Magnesia  fMgO) 

3.14 
1.40 
0  42 

3.44 
1.28 
0  44 

0.00 
0.25 
0  02 

0.00 
83.66 
95  65 

100.00 
16.34 
4  35 

Potash  (K2O)     
Soda  (Na20) 

5.56 
7.12 

6.26 
2.65 

O.OO1 
4.79 

100.00 
34.01 

0.00 
65.99 

Ignition     

4.33 

7.79 

O.OO1 

100.00 

0.00 

98.28% 

99.77% 

10.26  % 

.... 

.... 

This  phonolite,  it  should  be  remarked,  consisted  essentially 
of  sanidin  feldspars  and  a  soda  zeolite,  together  with  accessory 
augite,  black  mica,  magnetic  and  titanic  iron,  and  possibly 
hauyne.  The  zeolite  is  assumed  to  have  originated  from  the  al- 
teration of  the  nepheline.  The  process  of  decomposition  would 
seem  to  consist,  then,  in  the  breaking  down  of  this  zeolite,  and 
the  conversion  of  the  rock  into  an  earthy  mass,  with  little  other 

1  Zeit.  der  Deutschen  Geol.  Gesellschaft,  Vol.  35,  1883,  p.  559. 

2  Gain.    The  calculations  for  potash  in  column  IV  gives:  107.79%  and  for 
ignition  164.77%. 


218  HOCK  DISINTEGRATION  AND   DECOMPOSITION 

change,  so  far  as  ultimate  composition  is  concerned,  than  a  loss 
of  a  considerable  proportion  of  its  soda,  and  an  assumption  of 
nearly  3.5  %  of  water.  The  decomposed  rock  yielded  55.44  % 
of  material  insoluble  in  hydrochloric  acid,  and  with  essentially 
the  composition  of  sanidin,  showing  that  this  mineral  underwent 
only  a  physical  disintegration,  the  decomposition  proper  being 
limited  to  the  other  constituents.1 

Turning  to  still  more  basic  rocks,  we  will  next  consider 
a  disintegrated  diabase  occurring  in  the  form  of  a  large  dike 
extending  from  Granite  Street  in  Somerville,  Massachusetts,  to 
Spot  Pond  in  Stoneham,  and  beyond.2  The  rock  at  the  point 
selected  for  study  (Medford)  is  a  coarsely  granular  admixture 
of  lath-shaped  feldspar,  black  mica,  augite,  and  brown  basaltic 
hornblende,  with  the  usual  sprinkling  of  apatite,  magnetite,  and 
ilmenite.  Secondary  uralite,  chlorite,  biotite,  leucoxene,  kaolin, 
calcite,  pyrite,  and  quartz  are  common.3 

The  rock  has  undergone  extensive  disintegration,  giving  rise 
to  loose  sand  and  gravel  of  a  deep  brown  color,  in  which  lie 
rounded  boulders  of  all  sizes  of  the  still  undecomposed  material. 
These  boulders,  as  is  usually  the  case,  show  a  more  or  less  con- 
centric structure,  from  without  inward,  until  a  solid  core  of 
unaltered  diabase  is  met  with.  (See  PI.  17,  and  Fig.  2,  PI.  20.) 

A  mechanical  separation  of  the  disintegrated  material  yielded 
results  as  below  :  — 


1. 

2. 
3. 
4. 

5. 
6. 

7. 
8. 
9. 
10. 

Coarse  gravel    ab 
Fine  gravel 
Coarse  sand 
Medium  sand 
Fine  sand 
Very  fine  sand 
Silt 
Fine  silt 
Clay 
Loss  at  110°  C. 

Dve            2     mm. 
2-1      mm. 
1-5     mm. 
.5-.25     mm. 
.25-.  1     mm. 
.1-.05     mm. 
.05-.  01      mm. 
.01-.005    mm. 
.005-.0001  mm. 

in  diameter 

42  300  °/ 

in  diameter  . 

.     .               20  355 

in  diameter  . 

12  723 

in  diameter  .     . 

.     .     .     .       9  567 

in  diameter  . 

4  907 

in  diameter  . 

.     .     .     .       4.181 

in  diameter  . 

1  128 

in  diameter  . 
in  diameter  . 

....       0.370 
.     .     .     .       1  670 

0.660 

11.     Loss  on  ignition 1.730 


99.691  % 

1  In  calculating  these  analyses,  it  was  found  that  the  loss  of  alumina  had 
exceeded  that  of  iron  oxide,  necessitating  the  assumption  of  the  last-named  as 
a  constant  for  comparison.     The  apparent  gain  in  potash  is  presumably  due  to 
errors  in  analysis,  since,  as  will  be  noted,  the  analysis  of  the  fresh  material,  given 
in  column  I,  foots  up  only  98.28%. 

2  See  Disintegration  and  Decomposition  of  Diabase  at  Medford,  Massachu- 
setts, by  G.  P.  Merrill,  Bull.  Geol.  Soc.  of  America,  Vol.  VII,  1896,  pp.  349-362. 

3  On  the  Petrographic  Characters  of  a  Dike  of  Diabase  in  the  Boston  Basin, 
by  W.  H.  Hobbs,  Bull.  Mus.  Comp.  Zoology,  Vol.  XVI,  No.  1,  1888. 


I 


WEATHERING    OF   DIABASE  219 

Of  the  above,  the  first  three  sizes  could  be  easily  recognized 
by  the  unaided  eyes,  as  composed  of  particles  of  a  compound 
nature.  In  number  4  the  separation  had  gone  a  trifle  farther, 
though  even  here  inspection  with  a  pocket  lens  revealed  the 
compound  nature  of  many  of  the  granules,  somewhat  obscured 
by  the  prevailing  discoloration  from  the  oxides  of  iron.  It 
forms  a  gray-brown  sand  composed  of  feldspathic  particles, 
dirty  brown  augites,  and  lustrous  scales  of  brown  mica.  Num- 
bers 5  and  6  seemed  composed  almost  wholly  of  beautifully 
lustrous,  dark  mahogany-brown  mica  scales,  while  7  would  pass 
for  a  finely  micaceous  umber.  Numbers  8  and  9  were  uni- 
formly ochreous,  the  last  being  several  shades  lighter  than 
number  8,  and  without  appreciable  grit. 

The  chemical  nature  of  the  fresh  and  decomposed  rock  is 
shown  in  the  accompanying  table,  the  results  being  in  nearly 
every  case  averages  obtained  from  two  or  more  analyses.  The 
"fresh"  material,  obtained  from  the  interior  of  one  of  the  boul- 
ders, is  firm  in  texture,  has  a  bright  clean  fracture,  and  shows  to 
the  unaided  eye  no  signs  of  decomposition.  When  pulverized 
and  treated  with  acid,  however,  it  effervesces  distinctly,  indi- 
cating the  presence  of  free  carbonates,  which  are  also  observ- 
able as  secondary  calcite  when  thin  sections  are  examined  under 
the  microscope.  Some  of  this  calcite  is  evidently  a  deposit  from 
infiltrated  waters,  being  derived  from  the  surrounding  decom- 
posed material,  while  a  portion  results  from  the  decomposition 
of  the  silicate  minerals  in  place.  Aside  from  a  slight  kaolini- 
zation  of  the  feldspars  and  development  of  chlorite  from  the 
ferruginous  silicates,  there  are  no  other  observable  signs  of  de- 
composition, though  the  presence  of  a  soda-bearing  zeolite  is  indi- 
cated by  cubes  of  chloride  of  sodium,  which  separate  out  when 
an  uncovered  slide  is  treated  with  a  drop  of  hydrochloric  acid. 

A  glance  at  this  table  is  sufficient  to  show  that  the  disinte- 
gration is  accompanied  by  decomposition  and  a  leaching  action 
which  has  resulted  in  the  removal  of  a  portion  of  the  more 
soluble  constituents.  The  fact  that  the  fresh  rock  yields  the 
larger  percentages  of  its  constituents  to  the  solvent  action  of 
acid  and  alkaline  solutions  is  readily  explained  on  this  ground, 
though  it  may  be  doubted  if  the  full  significance  of  the  fact,  so 
far  as  it  relates  to  siliceous  crystallines,  is  as  yet  appreciated. 
It  will  be  observed  that  36.23  %  of  the  fresh  rock  and  32.28% 
of  the  decomposed  is  thus  extracted. 


220  ROCK  DISINTEGRATION  AND   DECOMPOSITION 

ANALYSES  OF  FRESH  AND  DISINTEGRATED  DIABASE  FROM  MEDFORD 


SILT   FROM   DISINTEGRATED 

FRESH  DIABASE 

DISINTEGRATED 
DIABASE 

DIABASE,  Nos.  VII,  VIII, 
AND    IX    OF    TABLE,    ON 

P.  218 

I 

II 

III 

IV 

V 

VI 

VII 

CONSTITUENTS 

.2 

i»g 

1-2  * 

s 

.2  ^ 

5 

•a 

a 

2  •§  —  ' 

"S3 
| 

•it 

III 

'o  £  "« 
a 

2  -u    C3 

t<81 

»•*•>         ci 

"33  a  a 

>>  0    c3 

S  c  « 
t^  g  O 

M 
PQ 

111 

W 

111 

111 

Us 

I 

Silica  (Si02) 

% 

% 

% 

% 

% 

% 

fin  HC1      ) 
tin  Na2Co3  ) 

47.28 

r  1.19 

I   9.66 

1  44.44 

f   0.85 
\    8.65 

0.47 
22.63 

J13.51 

36.61 

Alumina  (Al20s) 

20.22 

4.74 

23.19 

4.86 

21.98 

Ferric  oxide 

(Fe203).     .     . 

3.66 

) 

5.88 

40.68 

Ferrous  oxide 

1  10.91 

12.70 

10.00 

12.83 

(FeO)     .    .     . 

8.89 

J 

Lime  (CaO)  .     . 

7.09 

3.09 

6.03 

1.50 

3.32 

0.12 

3.44 

Magnesia  (MgO) 

3.17 

2.20 

2.82 

1.84 

3.23 

0.79 

4.02 

Manganese  oxide 

MnO.     .     .     . 

0.77 

Not  det. 

0.52 

Not  det. 

Not  det. 

Not  det. 

Not  det. 

Potash  (K20)     . 

2.16 

1.21 

1.75 

0.68 

1.30 

0.52 

1.82 

Soda  (Na20)  .     . 

3.94 

0.50 

3.93 

0.17 

0.90 

1.24 

2.14 

Phosphoric  acid 

(P206)   .     .     . 

0.68 

Not  det. 

0.70 

Not  det. 

Not  det. 

Not  det. 

.... 

Ignition     .     .     . 

2.73 

2.73 

3.73 

3.73 

10.86 

0.11 

10.97 

100.59 

36.23 

99.81 

32.28 

77.52 

22.17 

99.68 

Of  the  material  classed  as  silt  in  columns  V,  VI,  and  VII,  or 
as  silt  and  clay,  on  p.  218,  and  which  constitutes  only  some 
3.17  %  of  the  entire  residual  debris,  77.87%  is  soluble  in  dilute 
hydrochloric  and  sodium  carbonate  solutions.  The  insoluble 
portion,  constituting  22.13%  of  the  silt,  consists  of  unaltered 
feldspar  and  iron,  lime  and  magnesian  silicates,  which  are  easily 
recognized  under  the  microscope,  in  the  form  of  minute,  sharply 
angular  particles.  Recalculating,  as  before,  the  matter  in  col- 
umns I  and  II  on  the  basis  of  100  and  considering  the  alumina 
as  a  constant  factor,  we  get  the  results  given  in  columns  VIII  to 
XII  inclusive,  representing,  so  far  as  it  can  be  obtained  by  this 


WEATHERING  OF   DIABASE 


221 


method,  the  actual  percentage  loss  of  materials  attending  the 
breaking  down. 

CALCULATED  Loss  or  MATERIAL 


VIII 

IX 

X 

XI 

XII 

RECALCULATED  ON 

J  8 

o  •*-* 

°    3 

CONSTITUENTS 

BASIS  OF  100 

73 

ti 

til 

fj- 

Fresh 

Decomposed 

8  w 

8  ,a  M 

fu-2 

Diabase 

Diabase 

q;     fr-i 

<n     c*    ^ 

£}       £*     +J 

P-i  «2 

cupa  § 

PH  W    § 

Silica  (Si02)    

47.01% 

44.51% 

8.48 

81.97% 

18.03% 

Alumina  (A1203)      .     .     . 

20.11 

23.24 

0.00 

100.00 

0.00 

Ferric  oxide  (Fe2O3)    .     . 
Ferrous  oxide  (FeO)     .     . 

3.63 

8.83 

1  12.71 

2.42 

81.90 

18.10 

Lime  (CaO)    

7.06 

6.04 

1.83 

74.11 

25.89 

Magnesia  (MgO) 

3.15 

2.85 

0.68 

78.30 

21.70 

Manganese  (MnO)  . 

0.77 

0.52 

0.32 

58.43 

41.57 

Potash  (K20)       .... 

2.14 

1.75 

0.62 

70.85 

29.15 

Soda  (Na20)  

3.91 

3.94 

0.50 

87.17 

12.83 

Phosphoric  acid  (P206)     . 

0.68 

0.70 

0.08 

88.61 

11.39 

Ignition 

2.71 

3.74 

0.00 

100.00 

0.00 

100.00  % 

100.00% 

14.93% 

.... 

.... 

From  the  figures  in  column  X  it  appears  that  there  has 
been  a  loss  of  some  14.93%  of  all  constituents.  The  increase 
in  water,  as  indicated  by  the  ignition,  is  a  natural  consequence 
of  hydration  and  the  presence  of  a  small  amount  of  organic 
matter.  This  increase,  it  should  be  stated,  is  greater  than  may 
be  at  first  apparent,  for  the  reason  that  the  fresh  rock  contains 
a  considerable  amount  of  secondary  calcite,  which  is  quite  lack- 
ing in  the  residual  sand.  A  large  part  of  the  ignition  in  col- 
umns I  and  VIII  is  therefore  to  be  accredited  to  carbonic  acid, 
and  not  to  water  of  hydration. 

From  columns  XI  and  XII  it  appears  that  of  all  the  essential 
constituents,  the  lime  and  potash  salts  have  suffered  the  most, 
though  the  iron  oxides  have  been  carried  away  to  the  amount 
of  18.10  %.  Magnesia  has  also  proven  very  susceptible  to  the 
solvent  action,  disappearing  to  the  amount  of  21.70%;  and 
lastly,  silica,  to  the  amount  of  18.03%.  The  small  original 
amounts  of  manganese  and  phosphoric  acid  render  the  results 


222 


KOCK  DISINTEGRATION  AND   DECOMPOSITION 


obtained  by  these  calculations  of  doubtful  value,  since  it  is  pos- 
sible they  may  be  due  to  errors  of  analysis. 

In  this  case,  as  in  that  of  the  granite  from  the  District 
of  Columbia,  we  have  to  do  with  only  the  earlier  stages  of  de- 
generation, with  conditions  which  are  as  much  in  the  nature 
of  mechanical  disintegration  as  of  chemical  decomposition.  As 
before,  then,  it  will  be  instructive  to  consider  cases  in  which,  in 
rocks  of  similar  nature,  the  decomposition  has  proceeded  much 
farther.  For  this  purpose  we  will  select  a  diabase  from  Spanish 
Guiana,1  and  basalts  from  Bohemia  and  the  Haute  Loire  as 
described  by  Ebelmen;2  in  each  instance  the  actual  analysis 
being  recalculated  to  the  basis  of  100. 


ANALYSES  OF  FRESH  AND  DECOMPOSED  DIABASE  FROM  SPANISH  GUIANA, 

VENEZUELA 


I 

II 

III 

IV 

v 

CONSTITUENTS 

1 

DECOMPOSED 

PERCENTAGE 
Loss  FOR  EN- 
TIRE EOCK 

PERCENTAGE  OF 
EACH  CONSTIT- 
UENT SAVED 

PERCENTAGE  OF 
EACH  CONSTIT- 
UENT LOST 

Silica  (Si02) 

49  35  °/ 

43  38  °/ 

20  92  °/ 

57  60  °/ 

42  40  °/ 

Alumina  (A1203)  .  .  . 
Ferric  iron  (Fe20s)  .  . 
Ferrous  iron  (FeO)  .  . 
Lime  (CaO)  
Magnesia  (MgO)  .  .  . 
Potash  (K20)  .... 
Soda  (Na20)  . 

15.30 

12.28 
9.60 
7.38 
0.85 
1  98 

18.36 
20.39 

2.37 
3.45 
0.59 
0  14 

3.27 
0.00 

8.05 
5.12 
0.33 
1  82 

78.62 
100.00 

16.17 
30.63 
54.12 
4  63 

21.38 
0.00 

83.23 
61.37 
45.88 
95  37 

Ignition  

3  25 

11  34 

0  00 

0  OO3 

0  00 

100.00  % 

100.00  % 

39.51  % 

.... 

.... 

In  the  case  of  the  diabase,  it  appears,  from  a  comparison 
of  the  figures  in  columns  I  and  III,  that  the  total  loss  of 
material  equals  39.51  %,  there  being  the  usual  gain  in  volatile 
matter. 

1  Quar.  Jour.  Geol.  Soc.  of  London,  Vol.  XXXV,  1879,  p.  586. 

2  Ann.  des  Mines,  Vol.  VII,  1845. 

3  Gain. 


WEATHERING   OF   DIABASE   AND   BASALT 


223 


ANALYSES  or  FRESH  AND  DECOMPOSED  BASALT  FROM  KAMMAR  BULL, 

BOHEMIA 


I 

II 

III 

IV 

V 

VI 

« 

K     ^ 

1 

fe 

h 

*~"   5 

fi     O 

H 

c    , 

CONSTITUENTS 

w 

H    0 

c  w 

EH 
X 

H 
W    H 

o 

^ 

^  a 

£   o 

g  I 

0     - 

<<    |    x 

<  5 

^          O? 

-  ^    & 

>  rX    "•; 

M 

^  P 

)-3       ? 

K   ^ 

N- 

M    - 

£ 

I  I 

J       £ 

5    § 

III 

K     < 

||| 

Silica  (Si02)    .... 

43.61 

43.00 

43.27   • 

15.04  loss 

67.01 

32.99 

Alumina  (A1203) 

12.26 

13.90 

18.13 

0.00    " 

100.00 

0.00 

Ferric  iron  (Fe2O3)   . 
Ferrous  iron  (FeO)  . 

3.51 
12.16 

5.40  \ 
8.30  / 

11.70 

9.10    " 

49.83 

50.17 

Lime  (CaO)     .... 

11.87 

12.10 

2.60 

9.60    " 

54.47 

84.53 

Magnesia  (MgO)  . 

9.14 

7.30 

3.40 

6.83    " 

25.90 

74.10 

Soda  (Na2O)    .... 
Potash  (K20)  .... 

2.721 
0.81  J 

0.50 

0.20 

3.39    " 

38.31 

61.69 

Water  (H2O)  .... 

4.42 

9.50 

20.70 

0.00 

100.00 



100.00  % 

100.00  % 

100.00  % 

43.96  loss 

ANALYSES  OF  FRESH  AND  DECOMPOSED  BASALT  FROM  CROUZET,  IN  THE 
HAUTE  LOIRE,  FRANCE 


I 

II 

III 

IV 

V 

o 

j. 

o 

c    , 

H 

H 

CONSTITUENTS 

g 

3 

K     y 
C     BS 

2  *  u 

1 

V. 

H    ^ 

||f 

III 

i 

8  o 

K    0 

Es3     ^     O 

W    •<    « 

K    <    § 

fa 

P  M 

CH  s:  w 

ft-  a  uj 

fc,  W  a 

Silica  (Si02)     .... 

48.29% 

37.09  % 

30.  34%  loss 

34.44% 

65.56% 

Alumina  (A1203)  .     .     . 

13.25 

30.75 

0.00      " 

100.00 

0.00 

Ferric  iron  (Fe203)    . 
Ferrous  iron  (FeO)    . 

0.00 
16.66 

4.31  •} 
0.00  J 

16.64      " 

11.16 

88.84 

Lime  (CaO)      .... 

7.33 

8.97 

3.46      " 

52.76 

47.24 

Magnesia  (MgO)   .     .     . 

7.03 

0.61 

6.77      " 

3.62 

96.38 

Potash  (K20)    .... 

1.81 

0.71 

1.51      " 

16.66 

83.34 

Soda  (Na2O)     .... 

2.71 

1.01 

1.40      " 

25.59 

74.41 

Ignition  

4.92 

16.55 

0.00 

100.00 

0.00 

100.00  % 

100.00% 

60.  12%  loss 

.... 

.... 

224  ROCK   DISINTEGRATION   AND   DECOMPOSITION 

Of  the  individual  constituents,  83.23%  of  the  original  lime, 
61.37  %  of  the  magnesia,  45.88%  of  the  potash,  95.37  %  of  the 
soda,  42.40  %  of  the  silica,  and  21.38  %  of  the  alumina  have  dis- 
appeared, the  calculations  being  made  on  a  Fe2O3  constant 
basis. 

In  the  case  of  the  Bohemian  basalt,  the  decomposition  com- 
menced with  the  formation  of  boulders,  which,  when  the 
process  had  not  gone  too  far,  still  showed  fresh,  unchanged 
basalt  interiorly,  but  became  more  and  more  altered  toward 
their  peripheries.  The  first  stage  of  decomposition  (column  II), 
it  will  be  noted,  consists,  aside  from  hydration,  in  a  slight  appar- 
ent loss  of  silica,  a  considerable  oxidation  of  the  iron  magnesia 
minerals,  accompanied  by  a  slight  loss  of  both  constituents,  and 
an  almost  complete  loss  of  alkalies.  In  the  second  stage  (column 
III)  lime  and  magnesia  are  both  lost  in  considerable  amounts, 
the  iron  passing  over  wholly  to  the  condition  of  sesquioxide,  and 
there  is  a  further  slight  diminution  in  the  proportional  amount 
of  silica.  It  is  evident  that  here  the  feldspars  were  the  first  of 
the  constituents  to  yield  to  the  decomposing  forces,  the  augite 
and  olivine  proving  most  refractory.  The  total  loss  of  material, 
it  will  be  noted,  amounts  to  43.96  %,  the  lime,  magnesia,  alka- 
lies, iron  oxides,  and  silica  disappearing  in  the  order  here 
mentioned. 

In  the  case  of  the  basalt  from  Crouzet,  the  analyses  show  a 
total  of  60.12%  loss,  or  over  one-half  of  the  original  material. 
This  loss  includes  nearly  two-thirds  of  the  original  silica, 
88.84%  of  the  iron,  and  96.38%  of  the  magnesia.  The  loss 
of  both  iron  and  magnesia  in  such  proportionally  large  quan- 
tities is  quite  unusual,  and  indicates,  so  far  as  the  iron  is  con- 
cerned, that  the  decomposition  took  place  under  conditions 
excluding  a  sufficient  supply  of  oxygen  to  convert  the  same 
into  the  insoluble  sesquioxide,  or  where  subjected  to  the 'de- 
oxidizing and  solvent  action  of  organic  acids.  The  removal  of 
the  magnesia,  which  must  have  existed  mainly  in  the  mineral 
olivine,  indicates  that  the  decomposition  has  gone  on  even  to 
the  production  of  carbonate  of  magnesia  and  the  separation 
of  free  silica  and  iron  oxides. 

An  analysis  by  the  present  writer  of  a  closely  related  rock, 
a  diorite,  and  its  residual  soil,  from  North  Garden,  Albemarle 
County,  Virginia,  yielded  the  results  given  in  columns  I  and 
II  below.  The  rock  here  was  fine-grained,  of  an  almost  coal- 


WEATHERING  OF  DIORITE 


225 


black  color  finely  speckled  with  whitish  flecks  due  to  the 
presence  of  feldspars.  The  microscope  showed  it  to  be  com- 
posed mainly  of  hornblende  with  interstitial  soda-lime  feldspars 
and  scattering  areas  of  titanic  iron.  The  clay,  or  soil,  to  which 
it  gave  rise  was  deep  brownish  red  in  color  and  highly  plastic, 
though  distinctly  gritty  from  the  presence  of  undecomposed 
minerals.  In  columns  III,  IV.  and  V  are  given  the  loss  and 
gain  of  the  various  constituents  calculated  on  an  alumina 
constant  basis,  as  before. 

ANALYSES  OF  FRESH  AND  DECOMPOSED  DIORITE  FROM  ALBEMARLE  COUNTY, 

VIRGINIA 


I 

II 

III 

IV 

V 

CONSTITUENTS 

FRESH 

DECOM- 
POSED 

CALCULATED 
Loss  FOR  EN- 

PER  CENT 
OF  EACH 
CONSTITU- 

PER CENT 
OF  EACH 

CONSTITU- 

ENT SAVED 

ENT  LOST 

Silica  (Si02)  

46.75% 

42.44  % 

17.  43%  loss 

62.69% 

37.31  % 

Alumina  (A1203)     .     .     . 

17.61 

25.51 

0.00 

100.00 

0.00 

Iron  sesquioxide  (Fe203)  l 

16.79 

19.20 

3.53 

78.97 

21.03 

Lime  (CaO)   

9.46 

0.37 

9.20 

2.70 

97.30 

Magnesia  (MgO)     .     . 

5.12 

0.21 

4.97 

2.83 

97.17 

Potash  (K20)      .... 

0.55 

0.49 

0.21 

61.25 

38.75 

Soda  (Na20)  . 

2  56 

0.56 

2.17 

15.13 

84.87 

Phosphoric  acid  (P20)5    . 

0.25 

0.29 

0.00 

80.11 

19.87 

Ignition      

0.92 

10.92 

0.00 

100.00 

0.00 

100.01  % 

99.99  % 

37.  51%  loss 

.... 

.... 

The  ultra  basic  rocks,  —  peridotites  and  pyroxenites,  —  from 
the  very  nature  of  their  composition,  must  yield  on  decompo- 
sition residues  poor  in  the  presence  of  alkalies  and  rich  in 
iron  or  aluminum  and  magnesian  compounds.  Owing,  further, 
to  their  poverty  in  alkali-bearing  silicates,  the  process  of  decom- 
position must  be  less  complex,  consisting  essentially  in  hydra- 
tion,  oxidation,  and  a  production  of  iron,  lime,  and  magnesian 
carbonates  and  a  liberation  of  chalcedonic  silica. 

During  the  process  these  rocks  as  a  rule  become  brownish, 
and,  on  the  surface,  often  irregularly  checked  with  a  fine  net- 
work of  rifts  which  become  filled  with  secondary  calcite,  mag- 
nesite,  and  chalcedony.  If  the  original  rock  is  an  olivme-rich 

1  All  iron  calculated  as  Fe2O3. 


226 


ROCK  DISINTEGRATION  AND   DECOMPOSITION 


peridotite,  these  clefts  may  become  filled  with  the  silicates  of 
nickel,  noumceite  and  garncerite,  which  may  be  of  sufficient 
abundance  to  form  valuable  ores.  This,  in  brief,  is  the  history 
of  the  nickel  ores  of  Riddles,  Oregon,  and  of  New  Caledonia, 
though  the  process  is  more  properly  a  form  of  hydrometamor- 
phism  than  weathering. 

The  deep  green  serpentines  of  Harford  County,  Maryland, 
weather  slowly  down  into  a  gray-brown  soil,  which  consists  of 
60.17%  silica,  10.40%  of  iron  oxides,  14.81%  of  alumina,  and 
only  7.23%  magnesia.  The  fresh  rock,  on  the  other  hand,  car- 
ries nearly  40  %  of  magnesia,  8.50%  iron  and  other  metallic 
oxides,  and  less  than  one-half  of  one  per  cent  of  alumina. 

Natural  joint  blocks  occur  in  which  the  preliminary  stages 
of  weathering  are  manifested -by  a  brown,  ferruginous,  though 
tough  and  hard,  vesicular  crust  of  from  a  millimetre  to  two  or 
more  centimetres'  thickness,  enclosing  the  slightly  hydrated  but 
otherwise  unchanged  material. 

ANALYSES  OF  FRESH  AND  DECOMPOSED  SOAPSTONE  (ALTERED  PYROXENITE) 


I 

II 

III 

IV 

V 

CONSTITUENTS 

M 
o 
o 
H 

1 
1 

KESIDUAL  SOIL 

PERCENTAGE  OF 
Loss  FOB  ENTIRE 
EOCK 

PERCENTAGE  OF 
EACH  CONSTIT- 
UENT SAVED 

PERCENTAGE  OF 
EACH  CONSTIT- 
UENT LOST 

Silica  (Si02) 

38  85  °/ 

38  82  °/ 

16  92  °/ 

56  42  °/ 

43  58  °/ 

Alumina  (A120S)  .  .  . 
Iron  sesquioxide  (Fe20s)  l 
Lime  (CaO)  
Magnesia  (MgO)  .  .  . 
Potash  (K2O)  .... 
Soda  (NaaO)  
Ignition 

12.77 
12.86 
6.12 
22.58 
0.19 
0.11 
6  52 

22.61 
13.33 
6.13 
9.52 
0.18 
0.20 
9  21 

0.00 
5.33 
2.66 
17.20 
9.03 
0.00 
1  32 

ciu.-*^   /O 

100.00 
58.52 
55.55 
23.81 
52.94 
100.00 
79  74 

0.00 
41.48 
44.45 
76.19 
47.05 
0.00 
20  26 

100.00  % 

100.00  % 

52.46  % 

In  columns  I  and  II  above  are  given  (I)  the  composition  of 
an  altered  pyroxenite  (soapstone)  from  Albemarle  County, 
Virginia,  and  (II)  a  residual  soil  derived  from  the  same,  the 


1  All  iron  calculated  as  Fe203. 


WEATHERING  OF  PYROXENITES 


227 


latter  being  of  a  dull,  oclireous,  brown-red  color,  somewhat 
lumpy,  but  with  no  appreciable  grit  when  rubbed  between  the 
thumb  and  fingers. 

The  fresh  rock  is  of  a  blue-gray  color,  close  texture,  and 
consists,  as  shown  by  the  microscope,  of  elongated  crystals  of 
colorless  tremolite,  with  folia  of  talc  and  chlorite,  and  occasional 
opaque  granules  of  chromic  iron.  The  general  petrologic  feat- 
ures are  those  of  an  altered  pyroxenite. 

Recalculated  as  before,  the  analyses  give  the  results  shown 
in  columns  III,  IV,  and  V. 

Total  loss  of  material  52.46%,  including  water  of  hydration. 
The  most  striking  feature  brought  out  is  the  fact  that  the  mag- 
nesia has  been  carried  away  in  greater  proportional  quantity 
than  has  the  lime.  A  like  result  was  noted  by  Ebelmen  in  his 
analyses  of  the  decomposed  basalts  of  Crouzet,  which  are  given 
on  p.  223. 

ANALYSES  OF  FRESH  AND  DECOMPOSED  SOAPSTONE,  FAIRFAX  COUNTY,  VIRGINIA 


I 

II 

III 

IV 

V 

CONSTITUENTS 

a 
1 

ej 
PM 

RESIDUAL  SOIL 

PERCENTAGE  OF 
Loss  FOB  ENTIRE 
KOCK 

PERCENTAGE  OF 
EACH  CONSTIT- 
UENT SAVED 

PERCENTAGE  OF 
EACH  CONSTIT- 
UENT LOST 

Silica  (SiOo) 

58  40  °/ 

54  84  °/ 

46  31  °/ 

20  70  °/ 

79  30  °/ 

Alumina  (A1203)     .     .     . 
Iron  oxides(FeO  and  Fe203) 
Lime  (CaO)   .     .     . 

}7.« 

0  00 

<jt.a-±  /0 

33.75 
0  00 

I.UL  /0 
0.00 

100.00 

0.00 

Magnesia  (MgO) 
Alkalies  (K2O  and  Na20) 
Ignition  (H20)    .... 

29.19 
0.00 
4.97 

4.36 
0.00 
7.05 

28.23 
3.41 

3.29 

31.28 

96.71 
68.72 

100.00  % 

100.00% 

77.95% 

A  varietal  form  of  this  same  rock  occurring  near  Fostoria  in 
Fairfax  County,  this  state,  is  thoroughly  decomposed  throughout 
nearly  the  entire  area  to  a  depth  of  twenty  or  more  feet.  The 
fresh  rock  is  composed  mainly  of  a  light  greenish,  almost  white 
talc,  with  sporadic  patches  of  chlorite  some  five  or  more  millime- 
tres in  diameter,  and  scattering  granules  of  iron  ores.  The 


228  ROCK   DISINTEGRATION   AND   DECOMPOSITION 

decomposed  material  is  dull  brownish  or  gray,  and  when  washed 
and  submitted  to  microscopic  examinations  is  found  to  consist 
almost  wholly  of  brown  and  yellow-brown  scales  of  talcose 
material,  intermingled  with  an  impalpable  silt,  composed  so  far 
as  determinable  of  talcose  and  chloritic  shreds.  It  is  wholly 
without  grit,  and  with  a  decided  soapy  or  greasy  feeling. 
Analyses  of  fresh  and  decomposed  material,  and  calculations  as 
already  given,  yielded  results  as  shown  in  table  on  p.  227. 

The  principles  involved  in  the  decomposition  of  fragmen- 
tal  and  crystalline  stratified  rocks  are  not  so  different  from 
those  we  have  been  discussing  as  to  call  for  detailed  considera- 
tion. It  is  well  to  note,  however,  that  the  materials  composing 
rocks  of  this  type  are  themselves  a  product  of  these  very  dis- 
integrating and  decomposing  agencies,  but  which  have  become 
consolidated  into  rock  masses  and  now,  once  more  in  the  infinite 
cycle  of  change,  are  undergoing  a  breaking  up.  It  follows  from 
the  very  nature  of  the  case  that  such  rocks,  with  the  exception 
of  the  purely  calcareous  varieties,  will  undergo  less  chemical 
change  than  do  those  we  have  been  discussing.  Their  feld- 
spathic  and  easily  decomposable  silicate  constituents  long  ago 
yielded  to  the  decomposing  processes,  and  were  largely  removed 
before  consolidation  took  place.  Thus,  most  sandstones  are 
composed  largely  of  quartzose  sand,  the  least  soluble  and  least 
changeable  product,  it  may  be,  of  many  a  previous  disintegra- 
tion. Hence,  the  processes  involved  in  the  degeneration  of  the 
sandstones,  shales,  and  argillites  are  largely  mechanical,  with 
the  exception  of  those  which  carry  a  feldspathic  or  calcareous 
cement.  In  these  last-named,  the  cementing  material  is  gradu- 
ally leached  away,  and  the  rock  becomes  susceptible  to  the  action 
of  frost,  or  falls  away  to  loose  sand  simply  through  loss  of  cohe- 
sion. Heusser  and  Claraz l  described  the  itacolumites  of  Brazil 
as  subject  to  this  mechanical  degeneration,  the  process  being 
characterized  by  fissuration,  succeeded  by  complete  disintegra- 
tion. Among  siliceous  sandstone  it  is  the  binding  constituent 
that  yields  first,  as  is  naturally  to  be  expected,  and  as  has  been 
shown  by  the  experiments  conducted  by  R.  Schutze.2 

The  rocks  grouped  under  the  name  of  argillites,  though  com- 
posed of  detrital  materials  from  pre-existing  rocks,  and  of  parti- 

1  Ann.  des  Mines,  5th,  Vol.  XVII,  1860. 

2  Ueber  Verwitterungsvorgange  bei  Krystallinisclien  u.  Sedimentargesteinen, 
Inaug.  Dissertation  der  Friedrich-Alexanders  Universitat,  Berlin,  1886. 


WEATHERING   OF   ARGILLITE 


229 


cles  reduced  to  an  extreme  degree  of  fineness,  are,  nevertheless, 
quite  variable  in  composition,  as  already  noted.  As  a  rule,  they 
are  among  the  most  indestructible  of  rocks,  and  on  breaking 
down  yield  only  clays  which  differ  from  the  original  argillites 
mainly  in  degree  of  hydratioii  and  condition  of  oxidation  of  the 
iron  and  other  metallic  constituents.  Those  argillites  which 
carry  appreciable  quantities  of  still  undecomposed  silicates,  par- 
ticularly alkali-bearing  varieties,  are,  of  course,  more  susceptible, 
other  things  being  equal,  as  texture,  fissility,  etc. 

The  deep  blue-black    argillites  of    Harford  County,  Mary- 
land, as  shown  in  the  analyses  given  below,  do  contain  very 


ANALYSES  OF  FRESH  AND  DECOMPOSED  ARGILLITE,  HARFORD  COUNTY, 

MARYLAND 


CONSTITUENTS 

I 

II 

III 

IV 

V 

FRESH  ARGILLITE 

KESIDUAL  CLAY 

PERCENTAGE  OF 
Loss  FOR  EN- 
TIRE EOCK 

PERCENTAGE  OF 
EACH  CONSTIT- 
UENT 6AVED 

PERCENTAGE  OF 

EACH  CONSTIT- 
UENT LOST 

Silica1  (Si02)  

44.15% 
30.84 
14.87 
0.48 
0.27 
4.36 
0.51 
4.49 

24.17% 
39.90 
17.61 
None 
0.25 
1.24 
0.23 
16.62 

25.34  % 
0.00 
1.23 
0.48 
0.08 
3.39 
0.33 
0.00 

42.43  % 
100.00 
91.22 
0.00 
71.84 
22.04 
0.36 
287.37 

57.57% 
0.00 
8.78 
100.00 
28.16 
77.95 
99.64 
None 

Alumina  fA!203) 

Iron  oxide  (FeO  and  Fe203)      .     . 
Lime  (CaO) 

Magnesia  (MgO) 

Potash  (K20) 

Soda  (Na2O)   
Ignition  (C  and  H20)  .... 

99.97  % 

100.02  % 

40.83% 

.... 

.... 

considerable  quantities  of  these  undecomposed  silicates,  and 
though  extremely  tough  and  enduring  from  a  human  stand- 
point, in  time  decompose  in  a  very  interesting  manner.  In  the 
field  these  rocks  are  found  standing  nearly,  if  not  quite,  verti- 
cally, that  is,  with  their  evident  cleavage  vertical,  and  form- 
ing steep,  high  ridges  flanked  by  valleys  carved  from  the  softer 
rocks  on  either  hand.  In  the  fresh  cuts  made  during  the  work 


1  With  traces  of  Ti02. 
phur ;  hence  no  pyrite. 


Manganese  in  traces,  but  not  determined.     No  sul- 


230  ROCK  DISINTEGRATION  AND   DECOMPOSITION 

of  stripping,  to  open  new  quarries,  the  sound  rock  is  found  over- 
lain by  a  ^variable  thickness  of  ferruginous  residual  clay.  Joint 
blocks  and  splinters  of  the  slate  scattered  through  this  clay,  in 
all  stages  of  decomposition  leave  no  doubt  as  to  its  origin. 
Blocks,  deep  velvety  black  on  the  interior,  are  surrounded  by 
a  crust  of  ochreous  brown-red  decomposition  product,  the  decay 
penetrating  irregularly  like  the  processes  of  oxidation  into  a 
piece  of  metal.  The  first  physical  indication  of  decay  is  shown 
by  a  softening  of  the  slate,  so  that  it  may  be  readily  scratched 
by  the  thumb  nail,  and  an  assumption  of  a  soapy  or  greasy  feel- 
ing, the  entire  mass  finally  passing  over  to  the  deep  red-brown 
unctuous  clay,  sufficiently  rich  in  iron  to  serve  as  a  low-grade 
ochre,  for  paints.  The  incidental  chemical  changes  are  surpris- 
ingly large,  as  shown  by  the  analyses  given  on  p.  229,  column  I 
being  an  average  of  two  analyses  of  the  black,  little  altered 
material  from  the  interior  of  one  of  these  blocks,  and  II  that  of 
the  residual  clay.  In  III,  IV,  and  V  are  given  the  calculated 
losses  of  constituents,  as  before. 

This  residual  clay,  when  boiled  with  hydrochloric  acid  and 
sodium  carbonate  solutions,  yielded  up  nearly  70%  of  its  matter 
to  these  solvents,  leaving  a  residue  which,  when  examined  under 
the  microscope,  shows  only  faint  yellow-brown  scale-like  par- 
ticles, rarely  over  a  tenth  of  a  millimetre  in  diameter,  acting 
very  faintly,  if  at  all,  on  polarized  light,  and  with  borders  often 
serrate,  through  corrosion,  though  this  latter  feature  may  be 
due,  in  part,  to  the  action  of  the  solvents  used. 

Among  siliceous  rocks  poor  in  alkalies  or  iron-bearing 
silicates  the  degeneration  is  mainly  disintegration,  though  a 
small  amount  of  silica,  existing  in  either  crystalline  or  chalce- 
donic  forms,  is  usually  lost  through  solution.  Thus  the  cherts 
of  southwest  Missouri  break  down  into  porous  friable  forms, 
sometimes  passing  into  the  condition  of  loose  powder,  or  again 
retaining  sufficient  tenacity  to  be  utilized  for  filter  discs  and 
tubes,  as  at  Seneca,  in  Newton  County. 

Analyses  of  fresh  and  altered  forms  of  this  material,  as  given 
by  Dr.  E.  O.  Hovey,1  show  no  differences  that  are  of  sufficient 
importance  to  warrant  us  in  assuming  any  of  them  as  the  direct 
cause  of  disintegration.  The  change  is  evidently  mainly  physi- 
cal, though  it  is  more  than  probable  that  a  certain  amount  of 
interstitial  silica  has  been  removed.  It  is,  of  course,  possible 
1  Appendix  A,  Vol.  VII,  Missouri  Geological  Survey,  1894,  pp.  727-739. 


WEATHERING   OF   CHERTS  231 

that  here,  as  in  other  forms  of  decomposition,  extensive  solution 
may  have  taken  place,  leaving  a  residue  which,  so  far  as  compo- 
sition is  concerned,  gives  no  clew  to  the  changes  which  have 
occurred.  Dr.  Penrose,  however,  describes l  a  process  of  chert 
decay,  or  more  properly  disintegration,  as  manifested  in  the 
Batesville  region  of  Arkansas,  in  which  the  cause  of  the  break- 
ing down  is  more  apparent.  There  are  two  stages  in  the  proc- 
ess, as  described  :  (1)  A  transition  into  a  light,  porous,  opaque, 
buff-colored  rock  of  the  consistency  of  ordinary  pressed  brick, 
and  (2)  into  an  impalpable  white  or  brown  powder,  locally 
known  as  a  polishing  powder.  This  second  stage  is  not  so  con- 
spicuous a  feature  as  the  first,  since  the  finer  materials  thus 
formed  are  carried  off  by  surface  waters.  The  white  residual 
powder  often  contains  masses  of  the  porous,  semi-decomposed 
rock,  the  latter  in  turn  encircling  kernels  of  hard,  unaltered 
chert.  Throughout  this  region,  the  cherts  (of  Carboniferous 
age)  are  generally  decomposed  into  the  condition  of  a  more 
or  less  porous  mass  to  all  depths  up  to  ten  or  more  feet. 
In  all  cases  the  disintegration  may  be  traced  to  the  removal, 
by  leaching,  of  a  small  amount  of  interstitial  carbonate  of 
lime. 

When  we  come  to  a  consideration  of  the  Calcareous  rocks, 
we  find,  almost  invariably,  the  chemical  agencies  of  degenera- 
tion preponderating  over  those  that  are  purely  physical.  In 
arid  regions,  and  with  granular  crystalline  types,  physical 
agencies  may  for  a  time  prevail,  but  as  a  rule  the  process 
is  largely  chemical,  and  notable  for  its  simplicity.  The  de- 
composition is  due  mainly  to  the  action  of  meteoric  waters 
trickling  over  the  surface,  or  filtering  through  cracks  and  crev- 
ices, under  ordinary  conditions  of  atmospheric  pressure  and 
atmospheric  temperature.  Hence  the  process  is  one  of  super- 
ficial solution,  and  the  incidental  chemical  processes  set  in 
motion,  as  in  the  feldspar-bearing  rocks,  are  almost  entirely 
lacking.  It  follows  that,  only  the  lime  carbonate  is  removed 
in  appreciable  quantities,  while  the  less  soluble  impurities  are 
left  to  accumulate  in  the  form  of  ferruginous  clays,  admixed 
with  quartzose  particles,  chert  nodules,  etc.  Since  in  many 
limestones  the  amount  of  these  constituents  is  reduced  to  a 
minimum,  even  perhaps  to  the  fraction  of  one  per  cent,  so  it 
happens  that  hundreds,  or  even  thousands  of  feet  of  strata  may 
1  Ann.  Rep.  Geol.  Survey  of  Arkansas,  Vol.  I,  1890. 


232 


KOCK  DISINTEGRATION  AND   DECOMPOSITION 


disappear  without  leaving  more  than  a  very  thin  coating  of  soil 
in  their  place. 

An  interesting  illustration  of  the  changes  taking  place  in  the 
decomposition  of  an  impure  Carboniferous  limestone  is  described 
by  Penrose  in  his  treatise  on  the  genesis  of  manganese  deposits.1 
The  stone  in  its  least  changed  condition  is  of  a  granular  crys- 
talline structure  and  dark  chocolate-brown  color.  The  residual 
clay  from  its  decomposition  is  a  trifle  darker,  highly  plastic, 
and  quite  impervious.  Below  are  given  the  analyses  of  (I)  the 
fresh  rock  and  (II)  the  clay,  both  being  taken  from  the  same 
pit,  the  latter  being  of  about  fifteen  feet  in  thickness  and  over- 
lain by  a  capping  of  chert,  which  reduced  to  a  minimum  the 
possibility  of  any  admixture  of  foreign  matter.  The  materials 
were  dried  at  a  temperature  of  110°  to  115°  C.  before  analyzing. 


ANALYSES  OF  FRESH  LIMESTONE  AND  ITS  RESIDUAL  CLAY 


I 

II 

III 

IV 

V 

5 

H 

o  g 

&, 
0  S 

0    H 

CONSTITUENTS 

fc 

o 

Si 

sig 

W    H 
111 

=  1 

to 

fc    0 

Q    ^    W 

|»  3 

oO    W 

w 

1  1  * 

III 

PH  ^ 

M 

PH  (-1  M 

fS  H   to 

Pi  fd    to 

Silica  (Si02) 

4.13  % 

33.69% 

0.00% 

100.00  % 

0  00  °/ 

I.J-U      JQ 

/l/   /O 

Vf.WW      ^ 

V.UU     IQ 

Alumina  (A1203)     .     . 

4.19 

30.30 

0.35 

88.65 

11.35 

Ferric  iron  (Fe2O3)      .     . 

2.35 

1.99 

2.13 

10.44 

89.56 

Manganic  oxide  (MnO)     . 

4.33 

14.98 

2.49 

42.41 

57.59 

Lime  (CaO)    

44.79 

3.91 

44.32 

1.07 

98.93 

Magnesia  (MgO)      .     .     . 

0.30 

0.26 

6.25 

10.62 

89.38 

Potash  (K20)      .... 

0.35 

0.96 

0.23 

33.63 

66.37 

Soda  (Na20)  

0.16 

0.61 

0.085 

46.74 

53.26 

Water  (H20).     .... 

2.26 

10.76 

0.95 

58.37 

41.63 

Carbonic  acid  (C02)     .     . 

34.10 

0.00 

34.10 

0.00 

100.00 

Phosphoric  acid  (P205) 

3.04 

2.54 

2.73 

10.24 

89.76 

100.00  % 

100.00% 

97.635% 

These  analyses  have  been  recalculated  in  the  same  manner  as 
before,  excepting  that  silica,  instead  of  alumina,  is  taken  as  the 
constant  factor.  This  for  the  reason  above  suggested.  It  is 
believed  that  one  is  safe  in  assuming  little  or  no  silica  is  lost 

1  Ann.  Rep.  Geol.  Survey  of  Arkansas,  1890,  p.  179. 


WEATHERING   OF   CALCAREOUS   ROCKS  233 

here  through  the  action  of  alkaline  carbonates,  since  the  alka- 
lies are  almost  wholly  lacking  in  the  fresh  rock,  and  a  large 
portion  of  the  silica  doubtless  exists  as  free  quartz.  Recalcu- 
lating, then,  in  the  same  manner  as  before,  but  on  a  silica  con- 
stant basis,  we  obtain  the  matter  in  columns  III,  IV,  and  V. 

These  columns  bring  to  light  some  unexpected  features,  not 
the  least  interesting  of  which  is  the  fact  that  the  residual  clay, 
in  spite  of  its  highly  hydrated  condition,  in  reality  contains 
scarcely  half  the  amount  of  water  it  would,  had  the  small  amount 
(2.26%)  in  the  original  limestone  been  allowed  to  accumulate 
without  loss.  A  more  important,  though  perhaps  more  to  be  ex- 
pected, feature  is  the  entire  removal  of  that  portion  of  the  lime 
which  existed  as  carbonate,  as  indicated  by  the  absence  of  car- 
bonic acid  in  the  clay.  It  will  be  noted  that  97.635  %  of  the 
entire  rock  mass  has  disappeared  through  leaching,  leaving  only 
2.365  %  to  accumulate  as  an  insoluble  residue  in  the  form  of  soil. 

This  leaching  out  of  the  lime  carbonates  and  the  accumula- 
tion of  insoluble  residues  is  a  strikingly  conspicuous  feature  in 
regions  abounding  in  limestone  caverns,  and  to  it  is  due  the 
tenaceous  ferruginous  clays  which  cover  their  floors.  So  rich 
indeed  are  some  of  these  residual  deposits  in  iron  oxide  that 
in  some  instances  they  are  locally  used  for  pigments,  under  the 
name  of  ochre  or  mineral  paint,  or  again,  where  occurring  in 
large  quantities,  as  ores  of  iron.  (See  p.  267.) 

It  is  possible  that  loosely  consolidated  beds  of  shell  limestone 
may  undergo  a  process  of  change,  perhaps  more  nearly  akin  to 
alteration  than  decomposition,  through  agencies  quite  different 
from  those  we  have  been  considering. 

Darwin,  it  will  be  remembered,  found  the  shells  in  the  raised 
sea-beaches  of  San  Lorenzo,  South  America,  altered  to  the  con- 
dition of  a  white  powder  without  trace  of  organic  structure,  and 
consisting  of  carbonate,  sulphate  and  chloride  of  lime  with  sul- 
phate and  chloride  of  sodium.  This  alteration  he  believed  to 
be  due  to  a  mutual  reaction  taking  place  between  the  original 
sodium  chloride  derived  from  the  sea-water  and  the  lime  car- 
bonate of  the  shells,  and  he  speaks  of  it  as  an  interesting  illus- 
tration of  the  fact  that  the  dry  climate  of  the  west  coast  of 
South  America  is  much  less  favorable  to  the  preservation  of 
shell  structures  than  would  be  a  moist  one  where  the  salt  would 
be  removed  too  rapidly  for  the  double  decomposition  to  be 
brought  about. 


234  BOCK  DISINTEGRATION  AND  DECOMPOSITION 

Resume.  —  Making  all  due  allowance  for  possible  sources  of 
error  in  our  methods,  there  are  certain  general  deductions  that 
may  be  safely  drawn.  Not,  it  may  be,  from  our  own  analyses 
alone,  but  from  numerous  others  as  found  in  existing  literature.1 

Let  us  briefly  review  the  subject  and  make  the  deductions 
accordingly. 

In  glancing  over  the  columns  of  our  analyses,  it  is  at  once 
apparent  that  hydration  is  an  important  factor,  the  amount  of 
water  increasing  rapidly  as  decomposition  advances.  In  the 
earlier  stages  of  degeneration  it  is  doubtless  the  most  important 
factor.  There  is,  moreover,  among  the  siliceous  crystalline 
rocks,  in  every  case  a  loss  in  silica,  a  greater  proportional  loss  in 
lime,  magnesia,  and  the  alkalies,  and  a  proportional  increase  in 
the  amounts  of  alumina  and  sometimes  of  iron  oxides,  though 
the  apparent  gain  may  in  some  cases  be  due  to  the  change  in 
condition  from  ferrous  to  ferric  oxide.  As  a  whole,  however, 
there  is  a  very  decided  loss  of  materials.  Among  siliceous 
crystalline  rocks,  this  loss,  so  far  as  shown  by  available  analyses 
and  calculations,  rarely  amounts  to  more  than  50  %  of  the  entire 
rock  mass.  Among  calcareous  rocks,  on  the  other  hand,  it  may, 
in  extreme  cases,  amount  to  even  99  % . 

Of  all  the  ordinary  essential  mineral  constituents  the  free 
quartz  is  the  most  refractory  toward  purely  chemical  agen- 
cies, and  the  amount  of  silica  lost  from  this  source  must  be 
small,  though  Sorby2  thinks  to  have  distinguished  chemically 
corroded  quartz  granules  in  some  of  the  sands  examined  by  him. 
It  is,  however,  safe  to  say  that  the  mineral  suffers  chiefly  from 
mechanical  disruption, — that  silica  in  any  rock  which  is  re- 
moved during  the  process  of  decomposition  comes  mainly  from 
the  silicates,  and  not  from  the  free  quartz.  According  to  Bis- 
chof,  and  as  shown  by  our  own  work,  the  silicates  in  any  rock 
that  are  most  readily  decomposed  are,  as  a  rule,  those  contain- 
ing protoxides  of  iron  and  manganese,  or  lime,  and  the  first 
indication  of  decomposition  is  signalled  by  a  ferruginous  dis- 
coloration and  the  appearance  of  calcite.  The  evidence  bearing 
upon  the  relative  durability  of  the  various  minerals  consti- 
tuting rocks  is,  however,  quite  conflicting  and  unsatisfactory. 
Doubtless  much  depends  on  local  conditions. 

1  See  especially  Roth's  Allegemeine  u.  Chemische  Geologic,  Vol.  Ill,  and  Ebel- 
men's  papers  in  Ann.  des  Mines,  Vols.  VII,  1845,  and  XII,  1847. 

2  Proc.  Geol.  Soc.  of  London,  1879. 


GENERAL   DEDUCTIONS  235 

Dana  observed1  that  in  the  decomposition  of  the  granitic 
rocks  of  the  Chilean  coast  the  feldspars  yielded  first,  becoming 
white  and  opaque  and  of  a  friable  earthy  appearance.  But  it 
should  be  noted  that  there  is  nothing  in  Professor  Dana's  de- 
scription to  show  that  this  change  may  not  have  been  a  purely 
physical  one,  and  due  to  the  splitting  up  of  the  feldspars  along 
cleavage  lines.  Fournet,  from  a  study  of  the  processes  of  kao- 
linization,  was  led  to  state  2  that  hornblende  yields  less  readily 
to  decomposing  forces  than  does  feldspar,  when  the  two  are 
associated  in  the  same  rock.  Becker,  however,  in  studying 
deep-seated  decomposition  in  the  Comstock  Lode  of  Nevada, 
arrived  at  a  precisely  opposite  conclusion,  the  feldspars  as  a 
whole  offering  more  resistance  than  the  augite,  hornblende,  or 
mica. 

The  present  writer  has  described3  thick  sheets  of  augite  por- 
phyrite  in  Gallatin  County,  Montana,  in  which  the  feldspathic 
disintegration  has  gone  on  so  far  that  the  mass  falls  away  to  a 
coarse  sand,  from  which  still  perfectly  outlined  crystals  of  coal- 
black  augites  may  be  gleaned  in  profusion.  This  last  is, 
however,  a  semi-arid  region,  and  the  process  thus  far  one  of 
disintegration  more  than  decomposition.  In  a  moist,  or  perhaps 
in  any-  climate,  minerals  consisting  essentially  of  silicates  of 
alumina  and  magnesia  are  less  liable  to  decomposition  than 
those  containing  considerable  proportions  of  iron  protoxides  or 
of  lime.  This  for  the  reason  that  the  first-named  are  scarcely 
at  all  affected  by  the  ordinary  atmospheric  agents  of  solution. 
Bischof  goes  so  far  as  to  say  that  the  silicate  of  alumina  is  not 
at  all  affected  by  carbonic  acid,  but  the  researches  of  Miiller,  to 
which  reference  has  been  made,  and  our  own  calculations  tend 
to  disprove  this.  Dana  states4  that  in  the  decomposition  of 
basalt,  on  the  island  of  Tahiti,  the  olivine  is  the  earliest  to  give 
way,  becoming  first  iridescent  and  finally  falling  away  to  a  soft, 
pulverulent,  ochreous  yellow  or  brown  powder.  The  compact 
base  of  the  rock  yielded  next,  the  augites  holding  out  until  the 
last.  Those  silicates  which  are  least  liable  to  atmospheric  de- 
composition are,  as  is  to  be  expected,  those  which  have  resulted 
from  the  alteration  of  less  stable  silicates,  as  serpentine  from 

1  Report,  Wilkes's  Exploring  Expedition,  Geology,  p.  578. 

2  Ann.  de  Chimie  et  de  Physique,  Vol.  LV,  1833,  p.  240. 
8  Bull,  U.  S.  Geol.  Survey,  No.  110,  1894. 

4  Op.  cit.,  p.  298. 


236  ROCK   DISINTEGRATION  AND   DECOMPOSITION 

olivine,  epidote  from  hornblende,  or  kaolin  from  feldspar,  etc. 
A  few  silicates  like  tourmaline  and  zircon,  or  garnet,  or  oxides 
like  rutile  and  magnetite,  or  the  salts  of  rarer  earths  like  mona- 
zite,  etc.,  are  scarcely  at  all  affected  by  any  of  the  ordinary 
agents  of  decomposition,  but  remain  in  the  form  of  residual 
sands  in  the  beds  of  streams,  from  whence  the  lighter,  more 
decomposed  material  is  removed  by  erosion. 

In  the  weathering  of  potash-feldspar  rocks  carrying  black 
mica,  the  latter  mineral  is  as  a  rule  the  first  to  give  way,  and  at 
times  almost  wholly  disappears.  With  basic  rocks,  on  the 
other  hand,  the  dark  mica  is  one  of  the  most  enduring  of  the 
constituents,  and  in  the  residual  sands  may  be  found  in  surpris- 
ingly large  proportions. 

In  the  kaolinized  gneisses  of  northern  Delaware,  the  biotite, 
as  a  rule,  is  in  an  advanced  stage  of  decomposition,  while  the 
small  amount  of  primary  muscovite  is  still  fresh  and  intact, 
retaining  all  its  original  lustre  and  elasticity. 

Among  the  feldspars  the  potash  varieties  are,  as  a  rule,  far 
more  refractory  than  the  soda-lime,  or  plagioclase  varieties. 
This  is  shown  not  merely  by  our  own  investigations,  but  by 
those  of  others  as  well.  Roth  shows1  from  analyses  of  fresh 
and  weathered  phonolites,  nepheline  basalts,  and  dolorites,  that 
the  loss  of  soda  is  almost  invariably  greater  than  that  of 
potash. 

In  the  coarse,  pegmatitic  dikes  of  Delaware  County,  Penn- 
sylvania, the  microcline  masses,  as  mined  for  pottery  purposes, 
are  beautifully  fresh  and  translucent,  while  the  associated  oligo- 
clase  is  snow-white  through  a  splitting  up  along  cleavage  lines 
and  partial  decomposition.  Where  thrown  out  upon  the  dumps, 
this  whitened  mineral  shortly  falls  away  to  fine  sand, 'resembling, 
.at  first  glance,  kaolin,  but  is  distinctly  gritty. 

Max  Geldmacher  noted2  that  in  the  weathering  of  quartz 
porphyry  oligoclase  always  gave  way  before  the  oligoclase. 

Indeed,  as  shown  in  our  analyses,  in  certain  phases  of  rock 
degeneration,  the  potash  feldspars  may  lose  very  little  by 
decomposition,  but  be  converted  into  the  condition  of  fine 
silt  merely  through  a  mechanical  splitting  up.  This  fact  will 
in  part  explain  the  relative  scarcity  of  free  potassium  salts 

1  Op.  cit.,  3d  ed.,  2d  Heft. 

2Beitrage  zur  Verwitterung  der  Porphyre,  Inaug.  Dissertation,  Konigl. 
Freidrich  Alexander  Universitat,  Leipzig,  1889. 


GENERAL  DEDUCTIONS 


237 


(carbonates,  sulphates,  and  nitrates)  as  compared  with  those  of 
soda.1 

The  chemical  processes  involved  in  this  feldspathic  decompo- 
sition are  of  sufficient  importance  to  warrant  further  discussion, 
even  though  it  may  involve  a  certain  amount  of  repetition  of 
what  has  gone  before. 

Berthier,  Forschammer,  Brogniart,2  Fournet,3  and  others  ex- 
plained more  than  fifty  years  ago  the  process  of  feldspathic  dis- 
integration through  the  breaking  up  of  its  complex  molecule 
into  alkaline  silicates  soluble  in  water,  and  aluminous  silicates 
which  are  insoluble.  The  loss  in  silica,  as  noted  above,  was 
supposed  to  be  due  to  the  removal,  by  solution,  of  these  alka- 
line silicates.  Ebelmen,4  however,  subsequently  showed  that 
silicate  minerals  poor  or  quite  lacking  in  alkalies  lost  a  portion 
of  their  silica  with  equal  facility,  as  is  also  shown  in  our  analy- 
ses of  pyroxenites  on  pp.  226  and  227.  He  accounted  for  this  on 
the  supposition  that  the  silica  set  free  —  in  a  nascent  state — was 
soluble  either  in  pure  water,  or  water  containing  carbonic  acid. 
Bischof  states  that  when  meteoric  waters  containing  carbonic 
acid  filter  through  rocks  containing  alkaline  silicates,  the  first 

1  An  oligoclase  occurring  in  a  tourmaline  granite  on  the  southern  slope  of 
Monte  Mulatto,  near  Predazzo,  undergoes,  according  to  Lemberg  (Zeit,  der  Deut. 
Geol.  Gesellschaft,  28,  1876),  a  much  more  rapid  decomposition  than  the  ortho- 
clase  with  which  it  is  associated,  and  gives  rise  to  a  green,  lustreless,  serpentine- 
like  product.  The  chemical  changes  incidental  to  the  alteration  are  as  shown  in 
the  following  tables,  I  being  the  fresh  oligoclase,  and  II  the  decomposition 
product. 


CONSTITUENTS 

I 

II 

Silica  (Si02)   

59.51% 

45.29  % 

Alumina  (A^Os)     

25.10 

25.68 

Iron  sesquioxide  (Fe203)     

1.08 

12.49 

Lime  (CaO) 

4.03 

0.52 

Magnesia  (M^O) 

Trace 

2.88 

Potash  (K^O)      .... 

2.10 

3.00 

Soda  (Na20)  .... 

7.26 

2.14 

Water  (H»0)      

0.92 

8.00 

100.00% 

100.00  % 

2  Arch  du  Museum,  Vol.  I,  1839  (cited  by  Ebelmen). 
8  Ann.  de  Chimie  et  de  Physique,  Vol.  LV,  1833. 
4  Ann.  des  Mines,  Vol.  VII,  1845. 


238  KOCK  DISINTEGRATION  AND   DECOMPOSITION 

action  consists  in  the  partial  decomposition  of  these  substances 
by  the  carbonic  acid  and  the  formation  of  alkaline  carbonates, 
which  are  dissolved.  If  the  water  thus  impregnated,  on  pene- 
trating further  below  the  surface,  comes  in  contact  with  cal- 
careous silicates,  another  change  will  take  place  consisting  of  a 
decomposition  and  replacement  of  these  calcareous  silicates  by 
the  alkaline  silicates,  and  a  removal  of  the  lime  set  free,  as  a 
carbonate,  provided  the  water  still  contains  a  sufficient  amount 
of  carbonic  acid.  This  replacing  process  and  the  retention  of 
the  alkaline  silicates  is  accounted  for  on  the  supposition  that, 
in  their  nascent  state,  they  form  new  combinations  with  the 
other  silicates  present,  while  the  lime  remains  as  a  carbonate  to 
be  removed  or  not,  as  the  case  may  be.  He  further  states  that 
the  alkaline  carbonates  originating  in  the  manner  described 
are  among  the  most  soluble  substances  known  ;  the  carbonate 
of  soda  requires  for  solution  only  six  times  its  weight  of  water 
at  ordinary  temperatures.  Silica,  on  the  other  hand,  even  in 
its  most  soluble  form,  requires  ten  thousand  times  its  weight  of 
water  for  solution.  If,  therefore,  the  decomposition  of  feld- 
spar by  such  carbonated  water  were  ever  so  energetic,  there 
would  be  sufficient  water  for  the  solution  of  the  carbonate  of 
soda  formed.  But  if  the  silica  separated  meanwhile  amounted 
to  more  than  Yolhro  °^  ^ne  wa^er  present,  the  excess  could  not 
be  dissolved,  but  would  remain  mixed  with  the  kaolin. 

The  case  is  very  different  when  the  decomposition  of  feldspar 
is  affected  by  fresh  water  containing  only  the  minute  quantity 
of  carbonic  acid  derived  from  the  atmosphere.  By  the  action 
of  such  water,  only  very  small  quantities  of  alkaline  carbonates 
are  formed  ;  consequently  it  is  possible  that  the  silica  separated 
at  the  same  time,  also  small' in  quantity,  may  find  enough  water 
for  solution.  In  such  cases  the  whole  of  this  silica  would  be 
removed  with  the  alkaline  carbonates,  and  pure  kaolin  would 
be  left.  Such  an  action  as  this  does  not,  however,  appear  to 
take  place  ;  for  the  purest  of  kaolin  nearly  always  contains  an 
admixture  of  quartz  sand,  or  of  free  silica  in  some  of  its  forms. 

K.  V.  Murakozy  has  shown1  that  in  the  decomposition  of 
rhyolite  from  Nagy-Mihaly,  the  sanidin  passes  into  kaolin  and 
opal,  the  latter  separating  out  as  hyalite  in  veins  or  impure 
concretionary  forms. 

It  follows  from  this  consideration  that  in  the  decomposition 
1  Abstract  by  F.  Becke,  Neues  Jahrbuch,  1894,  1  Band,  2  Heft,  p.  291. 


GENERAL   DEDUCTIONS  239 

of  feldspar  into  kaolin  more  of  the  silica  separated  remains 
mixed  with  the  kaolin  formed,  the  greater  the  quantity  of 
carbonic  acid  in  water,  and  that,  perhaps,  the  amount  of  car- 
bonic acid  is  never  so  small  that  the  whole  of  the  silica  sep- 
arated in  the  decomposition  of  feldspar  can  be  removed.1  The 
above,  however,  overlooks  the  possible  presence  of  nitrates,  such 
as  we  now  know  from  the  researches  noted  on  p.  203  may  in 
many  cases  exist,  even  though  in  extremely  small  proportions. 
It  is  probable  that  the  small  amount  of  nitric  acid  formed 
by  the  bacteria  would,  if  not  taken  up  by  plant  growth,  com- 
bine immediately  with  the  alkalies,  forming  nitrates  which, 
owing  to  their  ready  solubility,  would  be  carried  away.  The 
larger  the  proportion  of  nitric  acid,  therefore,  the  greater 
would  be  the  amount  of  silica  intermingled  with  the  kaolin, 
since  whatever  proportion  of  the  alkalies  failed  to  be  carried 
away  as  nitrates  would  pretty  certainly  disappear  'as  carbo- 
nate. There  is  also  the  possibility,  especially  in  the  rocks 
rich  in  iron  protoxides,  that  a  portion  of  the  silica  may  com- 
bine with  the  iron,  as  already  noted. 

In  cases  where  the  decomposition  takes  place  under  the 
influence  of  a  sufficient  supply  of  oxygen,  all  iron,  and  presum- 
ably the  manganese  as  well,  would  be  converted  into  the  in- 
soluble hydrous  sesquioxide  form  and  remain  with  the  residue. 
Where,  however,  the  supply  of  oxygen  is  insufficient,  a  por- 
tion or  all  of  thesev  constituents  may  be  removed  in  the  form 
of  protoxide  carbonates,  or,  in  the  case  of  iron,  of  a  ferrous 
sulphate.  These  facts  well  account  for  the  variation  in  sta- 
bility of  the  iron,  as  indicated  in  the  preceding  analyses. 

Reference  has  already  been  made  to  the  fact  that  the  mag- 
nesia from  the  decomposition  of  magnesian  silicates  was  some- 
times removed  in  greater  relative  portions  than  was  the  lime. 
This  seeming  anomaly  is  also  sometimes  met  with  in  cal- 
careous stratified  rocks.  Roth2  showed  that  in  the  weather- 
ing of  dolomitic  limestones,  the  magnesia  is  often  removed  in 
greater  proportional  quantities  than  the  more  soluble  lime 
carbonate. 

The  researches  of  Hitterman  3  showed,  however,  that  carbonic 

1  Chemical  and  Physical  Geology,  by  Gustav  Bischof,  Vol.  II,  pp.  182,  183. 
*Op.  cit.,  Vol.  III. 

3  Die  Verwitterungeproducte  von  Gesteinen  der  Triasformation  Frankers, 
Inaug.  Dissertation,  Freidrich-Alexanders  Universitat,  Munich,  1889. 


240  ROCK   DISINTEGRATION  AND   DECOMPOSITION 

acid  solutions  may  exert  a  scarcely  appreciable  effect  upon  mag- 
nesian  carbonate,  which  therefore  accumulates  in  the  residual 
soils. 

It  is  safe  to  say  that  while  the  general  process  of  rock- 
weathering  may  be  quite  simple,  as  outlined,  there  are  many 
minor  reactions  which  it  is  not  possible  to  describe  in  detail. 

It  has  been  shown  that  even  in  firm  rocks  a  mutual  chemi- 
cal reaction  is  not  uncommon  among  minerals  lying  in  close 
juxtaposition,  giving  rise  to  what  are  known  as  reaction  rims 
or  zones  composed  of  secondary  minerals.  This  is  a  par- 
ticularly conspicuous  feature  in  many  gabbros,  where  olivine 
and  feldspar  are  closely  adjacent.  In  these  cases,  a  mutual 
interchange  of  elements  may  take  place,  giving  rise  to  garnets, 
free  quartz,  or  other  minerals  as  the  case  may  be.  This  is, 
to  be  sure,  a  deep-seated  change,  to  be  classed  as  alteration 
rather  than  decomposition,  and  taking  place  presumably  under 
conditions  of  temperature  and  solution  quite  at  variance  with 
those  existing  on  the  immediate  surface.  It  is,  nevertheless, 
self-evident  that  when  elements  are  set  free  through  any 
process,  they  must  almost  immediately  recombine,  taking  those 
forms  which  existing  circumstances  may  dictate  and  that  close 
contact  of  particles  would  be  favorable  to  the  more  rapid  for- 
mation of  new  compounds.  In  a  mass  of  decomposing  rock, 
circumstances  are  almost  continually  changing,  and  the  infer- 
ence is  fair  that  new  combinations  are  continually  being  made 
and  unmade,  the  intricacies  of  which  we  are  unable  to  follow. 


PLATE   18 


FIG.  1.  Exfoliated  granite  in  the  Sierras. 

FIG.  2.   Talus  slopes  on  Pike's  Peak. 

FIG.  3.   Disintegrated  granite,  Ute  Pass,  Colorado. 


THE    WEATHERING-    OP    BOCKS   (Continued} 
III.     THE    PHYSICAL    MANIFESTATIONS 

Rock-weathering  manifests  itself  in  a  great  variety  of  ways, 
much  depending  upon  climate,  though  naturally  the  controlling 
factor  is  that  of  mineral  composition.  The  manner  of  weather- 
ing is  often  sufficiently  characteristic  to  be  of  great  importance 
in  determining  surface  contours,  as  well  as  incidentally  afford- 
ing a  means  for  the  identification  of  rock  masses  when  the 
outcrops  themselves  are  obscured  by  decomposition  products. 
Such  a  means  is  of  only  local  importance,  however,  since  under 
varying  conditions  the  resultant  forms  assumed,  even  by  similar 
rocks,  are  themselves  quite  variable.  It  is,  nevertheless,  not 
without  interest  to  note  the  varying  phases  of  weathering  in 
different  kinds  of  rocks,  the  incidental  contours  assumed,  the 
character  of  the  resultant  debris,  and,  at  the  same  time,  the 
controlling  forces  that  have  -been  instrumental  in  bringing 
about  the  final  result. 

(1)  Disintegration  without  Decomposition.  —  That  in  weather- 
ing, physical  and  chemical  agencies  may  go  on  either  singly  or 
conjointly  has  been  noted  in  previous  pages.  In  the  case  of 
single  minerals,  the  preliminary  disintegration  is  beautifully 
illustrated  in  the  large  oligoclase  masses  associated  with  micro- 
cline  in  the  feldspar  mines  of  Delaware  County,  Pennsylvania. 
In  the  dumps  of  waste  about  the  mines  these  are  found,  in  all 
stages  of  disintegration,  the  mineral  splitting  up  along  cleavage 
lines,  becoming  snow-white,  and  ultimately  falling  away  to  a 
kaolin-like  product,  but  which,  when  submitted  to  microscopic 
examination,  is  found  to  be  made  up  of  sharply  angular  cleavage 
particles,  showing  no  sign  of  decomposition  other  than  that  in- 
dicated by  occasional  opacity.  In  the  analyses  given  below  are 
shown  (I)  the  composition  of  a  fresh  oligoclase  (as  given  by 
Dana)  from  near  Wilmington,  Delaware,  (II)  the  snow-white 
R  241 


242       THE   PHYSICAL   MANIFESTATIONS   OF   WEATHERING 


cleaved,  but  still  moderately  firm  mineral  mentioned  above,  and 
(III)  the  flour-like  or  kaolin-like  product. 


I 

II 

III 

CONSTITUENTS 

FEESH  OLIGOCLASE 

OPAQUE  WHITE, 

BUT   STILL   FlEM 

OLIGOCLASE 

FINE  DUST  FROM 
DISINTEGRATED 

OLIGOOLASE 

Si02      

64.75% 

61.23  % 

56.73% 

A1203    

23.56 

25.65 

28.44 

CaO. 

2  84 

2  37 

2  95 

K2O  

1  11 

0.72 

1  12 

Na20     

9.04 

7.66 

5  81 

Ignition 

1  00 

5  67 

101.33% 

99.63% 

100.72% 

The  fact  that  granitic  and  gneissic  rocks  may  undergo  ex- 
tensive disintegration  with  slight  decomposition,  even  in  a 
moist  climate,  was  noted  by  Nordenskiold 1  in  Ceylon.  He 
says :  ' '  The  boundary  between  the  un weathered  granite  and 
that  which  has  been  converted  into  sand  is  often  so  sharp  that 
a  stroke  of  the  hammer  separates  the  crust  of  granitic  sand 
from  the  granite  blocks.  They  have  an  almost  fresh  surface, 
and  a  couple  of  millimetres  within  the  boundary  the  rock  is  quite 
unaltered.  No  formation  of  clay  takes  place  and  the  alteration 
to  which  the  rocks  are  subjected,  therefore,  consists  in  a  crum- 
bling or  formation  of  sand,  and  not,  or  at  least  only  to  a  very 
small  extent,  in  a  chemical  change.  At  every  road  section 
between  Galle,  Colombo,  and  Ratnapoora  the  granite  and  gneiss 
crumbled  down  to  a  coarse  sand,  which  was  again  bound  to- 
gether by  newly  formed  hydrated  peroxide  of  iron  to  a  peculiar 
porous  sandstone,  called  by  the  natives  cabook.2  This  sandstone 
forms  the  layer  lying  next  the  rock  in  nearly  all  the  hills  on  that 
part  of  the  island  which  we  visited.  It  evidently  belongs  to 
an  earlier  geological  period  than  the  Quaternary,  for  it  is  older 
than  the  recent  formation  of  valleys  and  rivers.  The  cabook 
often  contains  large,  rounded,  unweathered  granite  blocks,  quite 
resembling  the  rolled  stone  blocks  in  Sweden.  In  this  way 

1  Voyage  of  the  Vega,  Vol.  II,  1881,  p.  420. 

2  Laterite  ?      It  seems  so  regarded  by  H.  F.  Alexander,  Trans.  Edinburgh 
Geol.  Society,  Vol.  II,  1869-74,  p.  113. 


WEATHERING   INFLUENCED   BY   STRUCTURE  243 

there  arises  at  places  where  the  cabook  stratum  has  again 
been  broken  up  and  washed  away  by  currents  of  water,  forma- 
tions which  are  so  bewildering,  like  the  ridges  (osars)  and  hills 
with  erratic  blocks  in  Sweden  and  Finland,  that  I  was  aston- 
ished when  I  saw  them." 

The  same  features  are  brought  out  in  the  previous  descrip- 
tions relative  to  the  weathering  of  the  granite  of  the  District 
of  Columbia,  the  diabase  of  Medford,  Massachusetts,  and  other 
localities  mentioned  in  these  pages.  (See  pp.  206  and  218.) 
This  tendency  toward  disintegration  without  decomposition  is 
exaggerated  among  coarsely  crystalline  rocks,  as  is  abundantly 
exemplified  in  the  rocks  of  the  Pike's  Peak  (Colorado)  area.  (See 
PL  18.)  Among  those  of  finer  grain,  particularly  the  quartz- 
free  varieties,  as  the  Fourche  Mountain  (Arkansas)  syenites, 
decomposition  may  follow  so  closely  on  disintegration  that  little 
or  no  sand  is  formed,  sound  fresh  rock  passing  within  the  space 
of  a  few  millimetres  into  the  condition  of  residual  clay.1 

(2)  Weathering  influenced  by  Crystalline  Structure.  — It  is  else- 
where observed  that,  other  things  being  equal,  a  coarsely  gran- 
ular rock  will  disintegrate  more  rapidly  than  one  of  finer  grain. 

Lone  Mountain,  one  of  the  high  eruptive  peaks  on  the  Avest 
side  of  the  Madison  valley  in  Montana,  presents  in  its  upper 
portions  all  the  features  of  a  volcanic  crater  broken  down  on 
one  side  by  the  lava  flow.  The  facts  of  the  case  are,  however, 
that  the  coarser  grained  central  portion  has  been  disintegrated, 
and  swept  by  wind  and  rain  into  the  valleys,  while  the  fine- 
grained, more  compact  outer  portions,  those  which  solidified  near 
the  line  of  contact  with  adjacent  rocks,  remain  intact.  Pro- 
fessor Bell 2  describes  an  interesting  case  of  this  kind  where  the 
coarsely  crystalline  central  portion  of  a  "  greenstone  "  dike  has 
yielded  more  readily  to  erosion  than  at  the  sides  and  afforded 
channel-way  for  the  Mattagami  River,  north  of  Lake  Huron,  in 
Canada.  The  gneiss  adjoining  the  dike  having  been  shattered, 
yielded  also  to  decomposing  agencies  and  forms  now  a  second 
parallel  channel  on  each  side  of  the  central  one.  "  Between 
them  the  finer  grained,  hard,  and  undecayed  4  greenstone '  con- 

1  Dr.  Max  Fesca  has  noted  that  the  granitic  rocks  of  Kai  province,  Japan, 
yield  on  decomposing  gravel,  sand,  and  clayey  loams,  while  those  rocks  poor  in 
quartz,  such  as  the  syenites,  give  rise  only  to  clays  (Abhandlungen  und  Erlau- 
terungen  zur  Agronomischen  Karte  de  Prov.  Kai,  Kaiserlich  Japanischen  Geo- 
logischen  Reichsanstalt,  1887). 

2  Bull.  Geol.  Soc.  of  America,  Vol.  V,  1894,  p.  364. 


244        THE  PHYSICAL   MANIFESTATIONS   OF   WEATHERING 

stituting  the  outer  portions  of  the  dike  rises  up  in  the  shape  of 
ridges  and  chains  of  islands,  so  that  the  river  flows  as  a  main, 
central  channel,  more  or  less  separated  from  the  smaller  lateral 
ones."  The  same  writer  describes  several  instances  in  which 
long  straight  valleys  in  the  Archaean  regions  of  Canada,  now 
occupied  by  straight  river  stretches,  long  narrow  lakes  or  inlets 
of  the  larger  lakes,  are  due  to  the  decay  and  removal  of  the 
wide  "  greenstone  "  dikes,  or  of  parallel  dikes  with  narrow  belts 
of  rock  between.  Long  Lake,  north  of  Lake  Superior,  some  52 
miles  in  length,  is  mentioned  as  typical  of  lakes  of  this  class. 

(3)  Weathering  influenced  by  Structure  of  Rock  Masses.  —  In 
any  rock  mass  weathering  is  greatly  augmented  by  lines  of 
weakness,  such  as  joint  and  bedding  planes,  since  these  furnish 
so  many  additional  points  of  attack.  In  homogeneous  massive 
rocks  the  rate  of  disintegration  is  retarded  by  a  lack  of  vulner- 
able points,  and  the  resultant  form  is  that  of  rounded  bosses 
such  as  are  shown  in  plate  1. 

As  a  rule,  however,  the  most  massive  of  rocks  are  traversed 
by  one  or  more  series  of  joints  (see  PI.  14)  whereby  they  are 


FIG.  17.  —  Showing  the  influence  of  joints  in  the  production  of  boulders. 

divided  up  into  rhomboidal  blocks  of  varying  sizes.  Even 
when  not  sufficiently  developed  to  be  conspicuous,  such  joints 
not  infrequently  exist  as  lines  of  weakness  along  which  moisture 
and  the  accompanying  agents  of  disintegration  make  their  way, 
gradually  rounding  the  corners  until  there  is  left  an  oval  mass 
of  which  the  so-called  "  niggerheads  "  of  the  gabbro  area  about 
Baltimore  are  typical  examples.  In  nearly  all  such  rocks  the 
exfoliation  and  decomposition  take  place  in  the  form  of  con- 
centric layers,  like  the  coatings  on  an  onion.  This  holds  true 
with  the  huge  granitic  bosses,  as  well  as  with  the  smaller  joint 
blocks,  and  has  been  argued  by  some  of  the  earlier  geologists 
as  indicative  of  an  original  concretionary  structure.  Such  an 


WEATHERING   INFLUENCED   BY   STRUCTURE 


245 


assumption  seems,  however,  wholly  uncalled  for.  If  the  block  or 
mass  is  reasonably  homogeneous,  the  agencies  of  decomposition 
will  penetrate  nearly  uniformly  from  all  exposed  surfaces,  pro- 
ducing an  exfoliation  nearly  parallel  to  that  surface,  and  the 
concentric  structure  is  inevitable,  as  was  long  ago  pointed  out 
by  Werner. 

In  some  cases  the  tendency  to  assume  the  boss-like  form  is 
accentuated  through  the  presence  of  joints  running  approxi- 
mately parallel  to  the  exposed  surface,  such  joints  as  give  rise 
to  the  step-like  arrangement  of  the  stone  so  frequently  seen  in 
granite  quarries.  Stone  Mountain,  Georgia,  an  immense  boss 
of  light  gray  granite  some  2  miles  long  by  1|  wide  and  650  feet 


9SSSS&.'. 

FIG.  18.  —  Exfoliation  of  granite. 


high,  owes  its  form,  apparently,  wholly  to  exfoliation  parallel 
to  pre-existing  lines  of  weakness.  The  entire  mass,  so  far  as 
exposed  by  quarrying  operations,  is  made  up  of  imbricated  sheets 
of  granite,  which,  of  unknown  thickness  beneath  the  surface, 
thin  out  to  mere  knife  edges  above,  like  shingles  on  a  roof. 
Through  prolonged  exposure  the  superficial  layers  have  become 
detached  from  the  parent  mass,  and  doubtless  hundreds  of  feet 
in  vertical  thickness  completely  disintegrated  and  swept  away. 
With  many  geologists  these  joints,  in  themselves,  would  be 
accepted  as  due  to  atmospheric  action.  In  the  writer's  present 
opinion  they  are,  however,  the  result  of  torsional  strains  and 
once  existing  are  lines  of  weakness  which  become  more  and 
more  pronounced  as  weathering  progresses.  The  boss-like  form 
is  therefore  incidental  and  consequent.  The  process  of  exfo- 
liation has,  in  the  case  mentioned,  been  productive  of  some 
peculiar  results  which  may  be  described  in  detail. 


246       THE   PHYSICAL   MANIFESTATIONS   OF   WEATHERING 


As  above  mentioned,  the  sheets  of  granite,  varying  from  a  few 
inches  to  several  feet  in  thickness,  conform  in  a  general  way  to 
the  present  surface  of  the  hill.  Constant  expansion  and  con- 
traction from  temperature  changes  have,  in  the  manner  already 
described,  so  expanded  these  sheets  that,  bound  at  the  sides, 
they  have  found  relief  in  an  upward  direction  where  resistance 
was  least,  and  risen  in  dome  or  roof  shaped  forms,  as  shown  in 
the  sketch.  The  weight  of  the  sheets  higher  up  the  slopes,  im- 
pinging upon  the  edges  of  those  below,  has  in  some  cases 
undoubtedly  aided  in  the  work,  but  the  larger  part  is  due  to 
simple  expansion,  such  as  was  referred  to  on  p.  180. 

These  ruptured  sheets  are  rarely  more  than  10  inches  thick, 
but  10  or  20  feet  in  diameter.  The  material,  though  quite 
fresh  appearing,  is  loosely  granular  and  friable,  easily  reduced 
to  sand.  In  a  few  instances  small  avalanches  have  been  caused 
by  the  giving  way  of  the  sheets  below  and  the  consequent  slid- 
ing down  of  those  above  through  lack  of  support.  (See  Fig.  18.) 
This  same  mass  of  granite  sometimes  shows  upon  its  surface 
peculiar  circular  depressions,  one  within  another,  separated  by 

intervening  ridges  of  low 
relief,  such  as  have  been 
described  in  a  much  more 
perfect  stage  of  develop- 
ment by  Dr.  Robert  Bell l 
in  the  Huronian  rocks  of 
Canada.  These,  as  shown 
in  Fig.  19  from  Bell's 
paper,  are  some  3  or  4 
feet  in  diameter  and  3  or 
4  inches  high.  The  cause 
of  this  form  of  weather- 
ing at  Stone  Mountain 
is  not  apparent,  though 

Bel1' Y1  *"  case/^rld' 

regards  it  as  induced  by 
FlG  19  an  original  concretionary 

structure. 

The  spheroidal  structure  so  frequently  seen  in  basaltic  rocks, 
and  as  typified  in  the  sphaeroidische  absonderung  of  German 
writers,  may  perhaps  be  due  to  an  original  spheroidal  ten- 
Geol.  Soc.  of  America,  Vol.  V,  1894,  p.  362. 


t       '  WEATHERING  INFLUENCED  BY   STRUCTURE 

dency  caused  by  cooling,1  but  a  very  large  proportion  of  the 
spheroidal  masses  so  typical  of  the  decomposition  of  massive 
rock  is,  as  already  suggested,  due  wholly  to  external  causes. 
W.  P.  Blake  in  1855  called  attention  to  this  form  of  disintegra- 
tion in  the  massive  sandstones  near  San  Francisco  (California) 
and  pointed  out  the  true  explanation.2 

This  sandstone  is  described  as  occurring  in  the  form  of  layers 
from  a  few  inches  to  6  and  8  feet  in  thickness,  alternating  with 
beds  of  slate  and  shale.  Down  to  a  depth  of  10  or  20  feet,  or 
to  the  limits  of  atmospheric  action,  all  the  beds  have  turned  from 
gray  to  rusty  brown  or  drab.  "  There  are,  however,  parts  of  the 
upper  beds  that  have  not  yet  been  reached  and  changed  by  de- 
composition; these  parts  are  found  in  the  condition  of  spherical 
or  ellipsoidal  masses,  from  which  the  weathered  parts  scale  off 
in  successive  crusts.  These  nuclei  have  the  appearance  of  great 
rounded  boulders,  and  have  accumulated  in  great  numbers  at 
the  base  of  the  cliff."  In  this  case  the  sandstone  is  composed 
mainly  of  grains  of  quartz  and  a  little  feldspar  cemented  by 
calcite,  the  disintegration  being  due  mainly  to  the  removal  of 
this  cement  by  percolating  water,  while  the  change  in  color  is 
doubtless  due  to  oxidizing  pyrite  or  ferrous  carbonate. 

The  effect  of  percolating  waters  is  not,  however,  always  im- 
mediately destructive.  Though  in  themselves  carrying  cement- 
ing materials,  or  causing  an  oxidation  of  the  iron  carbonates  or 
sulphides,  a  local  induration  may  be  induced  along  the  joint 
lines  such  as  becomes  conspicuous  only  through  the  weathering 
away  of  the  non-indurated  portions.  Resultant  forms  may  be 
extremely  regular  or  again  irregular,  according  to  the  character 
of  the  lines  along  which  percolation  takes  place,  and  that  of  the 
rock  itself.  An  interesting  illustration  of  this  form  of  weather- 
ing is  that  given  by  Wyville  Thompson  3  as  occurring  on  the 
islands  of  Bermuda. 

"  This  dissolving  and  hardening  process,"  he  writes,  "  takes 
place  irregularly,  the  water  apparently  following  certain  courses 
in  its  percolations,  which  it  keeps  open,  and  the  walls  of  which 
it  hardens ;  and  in  consequence  of  this,  the  rock  weathers  most 
unequally,  leaving  extraordinary  rugged  fissures  and  pinnacles, 

1  T.  G.  Bonney,  Quar.  Jour.  Geol.  Soc.  of  London,  Vol.  XXXII,  1876,  p.  153. 

2  Expl.  and  Survey  for  a  Railroad  from  the  Mississippi  to  the  Pacific  Ocean; 
Report  on  the  Geology  of  the  Route,  near  the  32d  Parallel,  by  W.  P.  Blake. 

3  See  The  Atlantic,  Vol.  I.     Also  Bull.  25,  U.  S.  National  Museum. 


248       THE   PHYSICAL   MANIFESTATIONS   OF   WEATHERING 

and  piled  up  boulders,  the  cores  of  masses  which  have  been 
eaten  away,  more  like  slags  or  cinders  than  blocks  of  limestone. 
The  ridges  between  Harrington  Sound  and  Castle  Harbor  are  a 
good  example  of  this.  It  is  like  a  rockery  of  the  most  irregular 
and  fantastic  style,  and  there  seems  to  be  something  specially 
productive  in  the  soil;  for  every  crack  and  crevice  is  filled  with 
the  most  luxuriant  vegetation,  mossing  over  the  stones  and  train- 
ing up  as  tier  upon  tier  of  climbers,  clinging  to  the  trees  and 
rocks.  Frequently  the  percolation  of  hardening  matter,  from 
some  cause  or  other,  only  affects  certain  parts  of  a  mass  of  rock, 
leaving  spaces  occupied  by  free  sand.  There  seems  to  be  little 
doubt  that  it  is  by  the  clearing  out  of  the  sand  from  such 
spaces,  either  by  the  action  of  running  fresh  water  or  by  that 
of  the  sea,  that  those  remarkable  caves  are  formed  which  add 
so  much  to  the  interest  of  the  Bermudas." 

A  form  of  weathering  due  to  similar  causes,  but  productive 
of  results  much  more  regular  in  arrangement,  is  shown  in 
Fig.  4,  PI.  20,  from  a  block  of  weathered  sandstone  in  the 
National  Museum.  The  original  joints  through  which  the 
waters  filtered  are  easily  recognized  in  the  sharp  straight  lines 
running  diagonally  across  the  specimen.  Blocks  of  fine  shale 
and  argillite,  in  their  incipient  stages  of  weathering,  often  show 
concentric  bands  of  varying  color,  due  to  the  oxidizing  effect 
of  water  percolating  inward  from  all  sides  of  the  natural  joints 
as  shown  in  Fig.  3,  PI.  20. 

In  stratified  rocks  there  is,  as  a  rule,  a  lack  of  homogeneity, 
certain  layers  being  more  porous  than  others,  or  containing 
mineral  constituents  more  susceptible  to  the  attacking  forces. 
Such  rocks,  therefore,  weather  unevenly,  and  give  rise  to  ex- 
ceedingly ragged  contours.  The  finely  fissile  schists  standing 
nearly  on  edge  along  the  coast  of  Casco  Bay,  in  Maine,  under 
the  combined  influence  of  wave  and  atmospheric  action,  weather 
into  peculiarly  fantastic  forms  resembling  nothing  more  than 
piles  of  old  lumber  in  which  the  multitudinous  channels  formed 
by  boring  coleopterous  larvse  have  become  irregularly  enlarged 
by  decay.  (See  Fig.  1,  PI.  19.)  The  numerous  quartz  veins  by 
which  these  schists  are  traversed  stand  out  in  bold  relief  until 
no  longer  supported  by  the  matrix,  when  they  fall  to  the  beach, 
where,  together  with  fragments  of  the  schist,  they  are  gradually 
reduced  to  pebbles  and  fine  sand. 

(4)  Weathering  influenced  by  Mineral  Composition.  — Although 


PLATE   19 


FIG.  1.   Weathered  schist,  coast  of  Cape  Elizabeth,  Maine. 
FIG.  2.   Sandstone  bored  by  bees.  FIG.  3.   Slab  of  glaciated  limestone. 


WEATHERING   INFLUENCED   BY   COMPOSITION  249 

the  soda-lime  feldspars  yield  to  the  decomposing  agencies  more 
readily  than  the  potash  varieties,  basic  eruptives  do  not  in  all 
cases  decompose  more  rapidly  than  the  granitic  rocks  into  which 
they  -are  intruded,  as  is  well  illustrated  in  some  of  the  glaciated 
areas  about  Boston,  where  small,  compact  dikes  form  low  ridges 
a  few  inches  above  the  surface  of  the  enclosing  granite.  Much 
seems  to  depend  upon  the  character  of  the  secondary  minerals 
which  have  been  generated  in  a  rock  at  some  period  prior  to 
its  decomposition  proper.  Thus  those  dikes  containing  so  large 
a  proportion  of  secondary  epidote  as  to  be  of  a  dull  greenish 
hue  are  almost  invariably  more  enduring  than  the  granites, 
while  those,  on  the  other  hand,  in  which  the  secondary  minerals 
are  largely  chlorite,  calcite,  and  zeolitic  compounds,  yield  to  the 
decomposing  agencies  more  readily.  Even  when  the  dike  as 
a  whole  gives  way,  the  presence  of  epiclotic  aggregates  fre- 
quently manifests  itself  in  protruding  knots  and  bunches  above 
the  corroded  surface.  Knots  caused  by  segregations  of  black 
tourmalines  stand  out  in  the  same  way  from  the  surface  of  the 
granite  boss  called  Stone  Mountain,  near  Altanta,  Georgia. 
Garnets,  staurolites,  quartz  veins,  and  other  of  the  less  easily 
decomposed  minerals  may  stand  out  in  like  manner  from  the 
surface  of  the  rocks  of  which  they  form  a  part. 

Granitic  and  other  complex  crystalline  granular  rocks  will,  on 
exposure,  sometimes  take  on  a  pitted  surface,  owing  to  the  re- 
moval of  the  more  easily  decomposed  materials.  The  boulders 
of  nepheline  syenite  in  the  glacial  drift  about  Portland,  Maine, 
are  thus  corroded  to  trie  depth  of  several  millimetres  through 
the  removal  of  the  granular  nepheline,  while  the  feldspars  and 
hornblendes  project  irregularly. 

Calcareous  rocks  containing  silicates,  like  the  amphiboles  or 
pyroxenes,  show  like  roughened  surfaces  due  to  the  dissolving 
away  of  the  calcareous  matter,  leaving  the  silicates  projecting, 
or,  as  is  the  case  with  some  of  the  tremolite-bearing  dolomites 
used  for  building,  may  become  pitted  by  the  dropping  out  of 
the  tremolite  as  the  calcareous  cement  gives  way.1 

Many  sandstones  become  likewise  roughened  through  the 
removal  of  a  portion  of  the  cementing  constituent,  leaving  the 
siliceous  granules  projecting.  In  the  coarsely  crystalline  lime- 
stones and  dolomites  the  solution  and  weathering  effects  are 
often  first  manifested  along  cleavage  lines  and  the  contacts  of 
1  As  in  the  U.  S.  Capitol  Building  at  Washington. 


250       THE   PHYSICAL  MANIFESTATIONS   OF   WEATHERING 

the  individual  granules,  as  may  be  observed  in  many  an  old 
tombstone  or  polished  column. 

Even  where  the  decomposition  is  almost  purely  chemical,  the 
corroded  surfaces  are .  peculiarly  irregular,  as  shown  in  PI.  17. 
This  feature  is  doubtless  due  to  some  imperceptible  difference  in 
the  texture  of  the  stone,  or  to  the  presence  of  joints  and  flaws 
which  give  direction  to  the  solvent  fluids.  Calcareous  rocks 
consisting  of  an  admixture  of  calcite  and  dolomite  crystals  may 
undergo  disintegration  through  a  complete  or  partial  removal  of 
the  calcite  granules  by  solution,  the  dolomite  remaining  almost 
untouched.  Certain  dolomitic  limestones  near  Stockton,  Min- 
nesota, have  been  described1  as  peculiarly  subject  to  this  form 
of  disintegration.  The  mass  of  the  rock  consists  of  dolomitic 
crystals  and  granule's,  but  often  interlaminated  with  narrow 
bands  of  calcite.  Through  the  removal  of  the  latter,  the 
stone  becomes  porous  and  its  degeneration  so  complete  that 
"  shovelfuls  of  loose  sand  consisting  of  dolomitic  rhombohedra 
can  be  taken  up." 

Fine-grained,  compact,  and,  seemingly  homogeneous  rocks 
may,  on  account  of  imperceptible  differences  in  composition 
and  structure,  weather  out  in  strikingly  irregular  and  peculiar 
forms.  Figure  2  on  PI.  15  is  that  of  a  limestone  fragment  from 
Harrisonburg,  Virginia.  The  resemblance  to  cuneiform  charac- 
ters Is  so  close  that  it  is  not  surprising  that  such  were  at  first 
supposed  to  be  of  human  origin. 

Massive  granitic  rocks  seemingly  of  quite  uniform  composi- 
tion will  sometimes  weather  very  irregularly,  giving  rise  to 
erven-like  cavities,  in  general  shape  resembling  the  pot-holes  in 
the  beds  of  streams.  Reusch  has  described 2  such  in  exposed 
faces  of  granite  ledges  on  the  island  of  Corsica,  the  holes 
extending  inward  horizontally,  or  sometimes  with  a  slight  up- 
ward tendency.  The  cause  of  this  is  not  apparent  from  the 
description  given,  but  it  is  presumably  due  to  slight  textural 
differences  such  as  are  not  readily  discernible  in  the  decom- 
posed rock. 

In  any  rock  consisting  of  a  variety  of  minerals,  disintegration 
is  likely  to  constitute  a  more  prominent  feature  of  weathering 
than  in  one  of  less  complexity  of  composition,  owing  to  the 
unequally  refractory  properties  of  its  constituents.  Thus  a 

1  Hall  and  Sardeson,  Bull.  Geol.  Soc.  of  America,  Vol.  VI,  1895,  p.  184. 

2  Eorhandlinger  i  Videnskabs-Selskabet  i  Christiania,  1878,  No.  7,  pp.  24-27. 


WEATHERING   INFLUENCED  BY   COMPOSITION  251 

granite  must  yield  a  sand,  while  a  purely  feldspathic,  pyrox- 
enic,  or  calcareous  rock  may  yield  only  clays. 

Beds  of  feldspathic  quartzite,  through  the  decomposition  of 
the  feldspar,  undergo  disintegration,  giving  rise  to  beds  of 
friable  siliceous  sand  interlaminated  with  kaolin,  as  described  by 
Dana.1  The  same  author  also  describes  an  interesting  pseudo- 
breccia  formed  by  a  quartzite  divided  up  by  a  succession  of 
cracks  into  which  limonite  from  decomposing  pyrite  has  fil- 
tered and  acted  as  a  colored  cement.  He  says  :  "  Many  of  the 
pieces  lie  in  place  barely  separated  from  one  another,  and  ap- 
pear to  be  undergoing  new  divisions.  But  in  the  lower  part, 
large  pieces  look^as  if  there  had  been  wide  displacements;  yet 
the  hardly  disturbed  condition  of  the  upper  half  proves  that 
the  apparent  displacement  is  due  to  the  extension  of  the  color- 
ing and  penetrating  limonite.  The  cracks  are  made  in  part 
by  the  extremely  slow,  wedge-like  action  of  the  depositing 
limonite." 

Heusser  and  Claraz2  describe  somewhat  similar  breccias 
formed  in  Brazil  through  the  weathering  of  crystalline  schists 
rich  in  iron.  These  breccias  consist  of. angular  fragments  of 
schist,  more  or  less  decomposed,  firmly  cemented  by  limonite. 

The  boulders  of  Oriskany  quartzite  in  the  Cretaceous  gravel 
about  Washington,  District  of  Columbia,  are  composed  of 
rounded  and  angular  quartz  fragments  tightly  bound  together 
by  a  fine  granular  crystalline  aggregate  of  quartz  and  feldspar. 
Disintegration  first  manifests  itself  on  the  exterior  of  the 
boulders  in  the  form  of  an  irregular  network  of  grooves  or 
channels,  which  gradually  become  more  and'  more  conspicuous 
until  the  boulder  falls  into  bluntly  pyramidal  fragments  and 
finally  into  sand.  The  microscope  shows  that  the  disintegra- 
tion is  due  wholly  to  the  disaggregation  and  partial  kaoliniza- 
tion  of  the  binding  constituents  whereby  all  cohesion  is  lost> 
and  disintegration  follows  from  necessity.  (Fig.  1,  PI-  20.) 

This  form  of  disintegration  seems  to  take  place  only  in 
boulders  exposed  at  or  near  the  surface,  and  is  believed  to  be 
due  primarily  to  expansion  and  contraction  from  alternations 
of  temperature. 

Many  rocks,  owing  to  a  lack  of  homogeneity,  weather  with 
extreme  irregularity  and  give  rise  to  odd  and  sometimes  fan- 

1  Am.  Jour,  of  Science,  Vol.  XXVIII,  1884. 

2  Ann.  des  Mines,  5th  Series,  Vol.  XVII,  18GO,  p.  290. 


252       THE   PHYSICAL  MANIFESTATIONS   OF   WEATHERING 

tastic  forms.  In  the  case  of  a  friable  sand  or  limestone,  sub- 
ject to  wind  or  rain  erosion  or  to  solution,  certain  portions  may 
be  protected  by  a  capping  of  other  rock  while  the  intervening 
material  is  carried  away.  There  thus  arise  spindle-shaped 
forms  of  varying  proportions,  each  capped  by  the  roof  or  hat- 
like  block  to  which  it  owes  its  origin.  Such  have  been  noted 
in  many  regions,  and  have  been  described  by  Hayden  as  occur- 
ring on  a  colossal  scale  in  Colorado.  In*  the  case  of  strata 
lying  nearly  horizontal,  it  rarely  happens  that  all  possess  the 
same  power  of  resistance,  the  more  friable  weathering  away 
with  the  greatest  rapidity,  leaving  the  harder  layers  for  a 
time  projecting  in  rib-like  masses,  to  ultimately  break  down 
in  large  angular  blocks  as  the  support  below  is  gradually 
removed.  Friable  beds  of  sedimentary  rock  are  thus  not  infre- 
quently protected  by  a  capping  of  impervious  lava.  Continual 
percolation  of  water  through  existing  joints  and  fractures  in 
time,  however,  erode  away,  in  part,  the  underlying  material, 
causing  the  landscape  to  assume  the  Table  Mountain  appear- 
ance, where  each  flat-topped  hill  represents  residual  masses  of 
a  once  continuous  plateau,  now  isolated  in  the  manner  described. 

(5)  Results  due  to  Position.  —  In  very  many  instances  loose 
blocks  of  stone  lying  exposed  upon  the  ground,  will  undergo 
a  more  rapid  disintegration  from  the  lower  surface,  a  feature 
evidently  due  to  the  fact  that  this  portion  of  the  rock  is  kept 
in  a  state  of  continual  moisture.  This  form  of  disintegration 
results  in  the  production  of  oval,  flattened,  scale-like  masses, 
quite  independent  of  the  original  jointing.  In  other  cases 
decomposition  going  on  from  all  exposed  sides  of  a  joint  block 
may  in  time  produce  the  so-called  rocking-stones  or  "  logans  " 
and  "tors"  of  English  writers,  though  some  of  these  are  un- 
doubtedly nicely  balanced  boulders  from  the  glacial  drift. 

A  mass  of  rock  may  be  prevented  from  undergoing  disinte- 
gration, even  though  partially  decomposed,  by  its  surroundings. 
Thus,  in  driving  the  tunnel  for  the  waterworks  extension,  in 
Washington,  natural  joint  blocks  of  hard  and  apparently  firm 
rock  brought  to  the  surface  would  fall  away  to  loose  sand  in 
course  of  a  few  days,  or  months,  as  the  case  "might  be,  much 
depending  on  the  conditions  of  the  weather  and  the  state  of 
decay.  This  characteristic  was  sufficiently  pronounced  to 
attract  even  the  attention  of  the  workmen,  who  described  the 
rock  as  "  slaking  "  and  believed  it  to  contain  quicklime. 


RESULTS   DUE   TO   POSITION  253 

The  fact  was  that  percolating  waters  had  brought  about  a 
partial  kaolinization  of  the  feldspar,  and  hydration,  without 
great  oxidation  of  the  iron-magnesian  constituent.  The  origi- 
nal pressure,  coupled  with  that  incidental  to  expansion  from 
hydration,  had,  however,  been  sufficient  to  hold  the  mass  intact 
until  exposed  briefly  to  atmospheric  influences. 

The  protective  action  of  water,  as  sometimes  shown  in  the 
beds  of  streams  and  in  deep  ravines,  may  be  only  apparent,  and 
due  to  the  fact  that  erosion  exceeds  decomposition,  the  stream 
having  cut  its  way  down  to  fresh  bed-rock.  Professor  Dana, 
to  be  sure,  writing  more  than  half  a  century  ago,1  described  the 
basaltic  rocks  of  Kiama,  Australia,  as  in  a  condition  of  advanced 
decomposition  except  where  protected  by  sea-water.  He  says  : 
"  It  is  a  general  and  important  fact  that  a  rock  which  alters 
rapidly  when  exposed  to  the  united  action  of  air  and  water,  is 
wholly  unchanged  when  immersed  in  water,  or  exposed  to  a 
constant  wetting  by  the  surf."  While  no  exception  can  be 
taken  to  the  conclusion  regarding  those  rocks  wholly  immersed, 
the  question  naturally  arises  in  one's  mind,  if  the  absence  of 
decomposition  products  in  those  rocks  constantly  wetted  by 
the  surf  and  in  many  stream  beds  may  not  be  due,  in  part  at 
least,  to  erosion.  That  rocks  so  situated  are  in  a  condition  far 
from  fresh,  is  well  known  to  any  petrologist  who  has  attempted 
to  gather  specimens. 

It  is  obvious  that  where  a  large  series  of  sedimentary  rocks 
composed,  it  may  be,  of  interbedded  limestones,  sandstones,  and 
argillites  are  turned  up  on  edge  and  exposed  alike  to  atmos- 
pheric agencies,  they  will  become  eroded  very  unequally.  If 
chemical  agencies  alone  prevail,  the  limestone  will  dwindle 
away  and  perhaps  give  rise  to  long  valleys  or  depressions 
walled  in  by  the  more  enduring  sands  and  shales,  and  carry- 
ing upon  its  bottom  a  fertile  clayey  soil  representing  not 
merely  the  insoluble  impurities  contained  by  the  original  lime- 
stone, but  also  the  mechanically  disintegrated  particles  washed 
in  from  the  hills  on  either  hand.  This  indeed  may  be  consid- 
ered the  history  of  the  fertile  Shenandoah  valley  of  Virginia, 
famous  alike  for  soft  contours,  beautiful  scenery,  and  the  exu- 
berant fertility  of  its  soils. 

When  stratified  rocks  lie  nearly  or  quite  horizontally,  much 
must  depend  upon  the  character  as  regards  permeability,  etc., 
1  Reports  of  Wilkes's  Exploring  Expedition,  Geology,  p.  614. 


254       THE  PHYSICAL  MANIFESTATIONS   OF   WEATHERING 

of  the  upper  layers,  since  these  may  so  protect  the  lower  lying 
as  to  retard  or  quite  stop  further  disintegration.  Further  than 
this,  an  easy  and  rapidly  disintegrating  superficial  layer  may 
yield  a  residual  clay  so  impervious  as  to  protect  the  underlying 
rocks  as  securely  as  a  mass  of  rock  itself,  or  so  hard  and  tough 
as  to  put  a  stop  to  purely  mechanical  erosion,  as  in  the  case  of 
the  laterite  beds  of  central  India. 

In  cases  where  thinly  bedded  rocks  lie  sharply  inclined,  it 
nearly  always  happens  that  certain  layers  decompose  more 
readily  than  others.  There  may  thus  arise  strikingly  ragged 
saw-tooth  contours,  the  more  enduring  layers  standing  out  in 
sharply  serrate  or  wall-like  masses,  while  the  softer  give  way 
and  become  obscured  by  their  own  debris. 

(6)  Induration  on  Exposure.  —  Many  rocks,  instead  of  becom- 
ing disintegrated  on  exposure,  undergo  a  kind  of  induration 
upon  the  exposed  surfaces.  This  is  particularly  the  case  with 
some  siliceous  sandstones.  The  water  with  which  the  stone  is 
permeated  holds  in  solution  certain  constituents,  as  silica,  car- 
bonate of  lime,  or  iron  oxides.  When  the  rock  is  so  situated 
that  this  "  quarry  water,"  as  it  is  popularly  called,  is  brought 
to  the  surface  and  evaporated,  it  binds  together  the  granules 
composing  the  stone,  forming  thus  a  more  or  less  superficial 
coating  of  a  more  enduring  nature.  The  induration  sometimes 
takes  place  so  rapidly  that  even  an  exposure  of  but  a  few  months 
is  sufficient  to  produce  very  marked  results  on  freshly  broken 
surfaces.  This  peculiarity  of  certain  classes  of  rocks  has  long 
been  known  to  quarrymen  and  stone  workers,  who  recognize 
the  fact  that  a  well-seasoned  stone  yields  much  less  readily  under 
the  chisel  than  one  that  is  newly  quarried.1 

A  somewhat  similar  induration,  due  to  purely  superficial 
causes,  has  been  described  by  Dr.  M.  E.  Wadsworth  as  taking 
place  on  the  surface  of  exposed  blocks  of  siliceous  sandstone  in 
Wisconsin.  "  The  St.  Peters  Sandstone  "  he  writes,2  "  is  com- 
posed almost  wholly  of  a  pure  quartz  sand,  and  in  the  outliers 
of  it  found  on  the  hilltops  south  of  the  town,  the  parts  covered 
by  the  soil  were  more  or  less  friable,  and  the  grains  distinct; 
while  the  exposed  portions  of  the  same  blocks  and  slabs  were 
greatly  indurated,  the  grains  almost  obliterated,  and  the  rock 
possessed  the  conchoidal  fracture  and  other  characteristics  of  a 

1  See  Stones  for  Building  and  Decoration,  p.  415. 

2  Proc.  Boston  Soc.  of  Natural  History,  Vol.  XXII,  1883,  p.  202. 


INDURATION  ON  EXPOSURE  255 

quartzite."  In  this  and  other  cases  cited  by  Dr.  Wadsworth, 
the  cementing  matter  is  silica. 

The  explanation  given  (in  letter  to  the  present  writer)  is  to 
the  effect  that  all  water,  including  that  of  rains,  as  well  as  ter- 
restrial, dissolves  silica,  which  is  again  deposited  under  suitable 
conditions.  Part  of  the  silica  apparently  comes  from  the  solu- 
tion of  the  quartz,  chalcedony,  and  opal,  and  a  part  from  the 
alteration  and  destruction  of  the  silicates.  Both  solution  and 
deposition  seem  at  times  to  take  place  on  the  immediate  surface, 
the  interior  waters  in  such  cases  playing  no  part. 

P.  Choffat  regards  it  as  possible  that  silica  set  free  through 
feldspathic  decomposition  in  granitic  rocks  may,  on  evaporation, 
be  redeposited  in  an  insoluble  form  in  the  interstices  of  the  fresh 
rock  in  the  immediate  vicinity,  thus  retarding  if  not  wholly 
preventing  further  decay  in  that  direction.1 

Professor  W.  O.  Crosby,  in  a  personal  memorandum  to  the 
writer,  calls  attention  to  the  fact  that  in  the  disintegrated 
granites  of  the  Pike's  Peak,  Colorado,  area,  the  rock  is  almost 
invariably  exceptionally  firm  and  impervious  along  the  joints, 
indicating  a  local  induration  due  perhaps  to  infiltration  of  iron 
oxides  or  silica.  Where  a  joint  face  bounds  a  ledge  of  rock,  it 
often  maintains  its  integrity,  weathering  out  in  relief  like  a 
quartz  vein,  while  the  granite  is  in  a  condition  of  advanced 
degeneration  all  around.  A  slight  break  in  the  face  of  a  joint 
plane,  in  such  cases,  may  lead  to  extensive  disintegration  behind 
it,  until  it  finally  falls  away  from  the  disintegrating  mass,  a  slab 
of  relatively  sound  rock. 

Andesitic  rocks  in  regions  of  limited  rainfall  have  been 
noted  by  Professor  G.  Vom  Rath  as  having  become  covered 
on  the  upper  surface  with  a  thin  layer  of  brown  iron  oxide, 
which  protected  them  from  further  disintegration.  Such 
crumbled  away  only  from  the  under  surfaces,  where  they  ab- 
sorbed moisture  from  the  ground,  and  gave  rise  thus  to  peculiar 
tent-like  and  mushroom-shaped  forms. 

The  present  writer  has  noted  in  the  Madison  valley,  north 
of  the  Yellowstone  Park,  rounded  masses  of  a  vesicular  rhyolite 
which  have,  through  the  same  causes,  been  reduced  to  the  con- 
dition of  mere  shells  with  openings  on  the  under  sides  and  that 

1  Sur  quelques  cas  d' erosion  atmospherique  dans  les  garnites  du  Minho,  Com- 
munica95es  da  Direc^ao  Dos  Trabalhos  Geologicos  de  Portugal,  Tome  3,  Fasc.  1, 
1895-96,  p.  17. 


256       THE   PHYSICAL  MANIFESTATIONS   OF   WEATHERING 

facing  the  direction  of  the  prevailing  winds.  In  these  cases, 
however,  the  wind  seemed  to  have  aided  their  formation,  not 
merely  through  transporting  the  disintegrated  material,  but  by 
catching  up  and  whirling  about  the  loosened  granules  within 
the  gradually  enlarging  cavity,  where,  by  force  of  impact,  as 
already  described,  they  became  themselves  agents  of  abrasion. 
Some  of  the  cavities  observed  were  of  sufficient  size  to  afford 
shelter  for  a  human  being  and  had  in  some  instances  served  as 
temporary  dens  for  wild  animals. 

Roth  mentions l  an  induration  evidently  somewhat  similar  to 
that  described  by  Vom  Rath  above,  as  having  taken  place,  on 
the  surface  of  a  reddish  yellow  sandstone  in  Fezzan,  North 
Africa.  The  crust  thus  formed  was  so  dense  and  hard  as  to 
break  with  a  shell-like  fracture  resembling  basalt.  A  similar 
incrustation  on  sandstone  from  the  Lydian  desert  was  found  to 
consist  of:  manganese  oxide,  30.57%  ;  iron  oxide,  36.86%  ; 
alumina,  8.91%  ;  silica,  8.44%  ;  barium  oxide,  4.89%  ;  sul- 
phuric acid,  4.06  %  ;  phosphoric  acid,  0.25  %  ;  and  water,  5.90  %. 

W.  P.  Blake  has  described  boulders  from  the  Colorado 
desert  colored  exteriorly  by  what  he  regarded  as  organic  matter 
received  from  water  during  a  period  of  submergence.  Similarly 
discolored  quartzitic  boulders  brought  by  G.  K.  Gilbert  from 
the  Sevier  desert  in  Utah,  and  examined  by  the  present  writer, 
show  a  thin  dark  varnish-like  coating,  not  inaptly  named  by  Mr. 
Gilbert  "  desert  varnish,"  and  which  consists  largely  of  oxides 
of  iron  and  manganese,  though  a  slight  amount  of  organic 
matter  is  present.  In  this  case  the  rock  is  composed  not  wholly 
of  quartz  granules,  but  carries  interstitial  calcite  and  feldspathic 
granules.  Near  the  discolored  surface  of  the  boulders  these  in- 
terstitial calcites  are  found  quite  dissolved  away,  leaving  cavities 
stained  by  a  dark  deposit  which  reacts  for  iron  and  manganese. 
Inasmuch  as  acid  solutions  obtained  from  fresh  and  uncolored 
portions  of  the  boulders  give  faint  reactions  of  the  same  nature, 
it  seems  very  probable  that  the  crust  is  due  to  a  concentration 
of  these  metals  in  a  condition  of  higher  oxidation  on  the  surface, 
whither  they  have  been  brought  by  capillarity,  while  the  more 
soluble  lime  carbonate  was  removed.2 

1  Allegemeine  u.  Chemische  Geologie,  2d  ed.,  Vol.  Ill,  p.  215. 

2  Although  such  discolorations  seem  to  have  been  noted  principally  in  desert 
regions,  they  are  by  no  means  limited  thereto.     The  quartzitic  boulders  in  the 
superficial  deposits  of  the  District  of  Columbia  show  at  times  a  like  discoloration, 
due  to  a  very  thin  coating  of  iron  and  manganese  oxide. 


INCIDENTAL   COLOR    CHANGES  257 

The  Potsdam  quartzites  of  Minnesota  have  had,  in  many  in- 
stances, an  almost  glass-like  polish  imparted  to  their  exposed 
surfaces  through  no  other  apparent  agency  than  that  of  wind- 
blown sand.  Unlike  a  polish  produced  by  artificial  methods, 
this  wind  polish  extends  to  the  bottoms  of  every  little  groove 
and  cavity,  or  over  every  protruding  knob  alike.  In  softer 
rocks,  or  rocks  of  less  homogeneous  structure,  the  same  agencies 
carve  out  the  softer  portions,  leaving  the  more  resisting  pro- 
truding, as  already  described  on  p.  186.  This  polish  is  so  per- 
fect, even  on  rough  surfaces,  as  to  suggest  a  partial  solution  of 
the  granules,  and  a  redeposition  of  the  dissolved  matter  in  the 
form  of  a  glaze,  but  the  microscope  proves  to  the  contrary. 
The  gloss  is  due  wholly  to  superficial  smoothing  and  has  no 
thickness  whatever,  nor  has  any  new  matter  been  deposited 
either  on  the  surface  or  between  the  granules. 

(7)  Changes  in  Color  incidental  to  Weathering.  —  That  in 
nearly  every  rock  a  change  in  color,  the  assumption  of  a 
brownish  or  reddish  hue,  is  an  early  indication  of  decomposition 
has  been  made  sufficiently  apparent  in  the  chapter  devoted  to  a 
discussion  of  the  chemical  changes  involved.  This  discolor- 
ation is,  however,  merely  incidental,  and  not  essential,  and  is 
found  to  diminish,  if  not  wholly  disappear,  as  the  distance  from 
the  surface  increases,  as  was  noted  in  the  case  of  the  granites  of 
the  District  of  Columbia  (p.  207)  and  the  diorites  of  the  Sierra 
Nevadas  (p.  274.  See  further  under  Color  of  Soils,  p.  385). 

Granitic  and  other  highly  feldspathic  rocks  carrying  pro- 
portionately small  amounts  of  iron  become  almost  invariably 
bleached  or  whitened  on  the  immediate  surface,  owing  in  part  to 
kaolinization  and  in  part  to  the  splitting  up  of  the  feldspars 
along  cleavage  lines. 

In  extreme  cases  rocks  consisting  of  an  admixture  of  feldspars 
and  iron-bearing  silicates,  but  in  which  the  first-named,  owing 
to  its  glassy  nature,  is  in  the  fresh  rock  quite  inconspicuous, 
become  almost  snow-white  in  the  earlier  stages  of  weathering. 
This,  as  in  the  case  above  mentioned,  is  due  to  the  change  in 
the  feldspars  and  the  consequent  obscuring  of  the  darker  sili- 
cates by  the  white  product  of  kaolinization.  Continued  decom- 
position must,  however,  attack  the  ferruginous  constituent  and 
the  usual  staining  ensue,  unless,  as  in  some  cases  possible,  suffi- 
cient carbonic  acid  may  exist  to  convert  the  iron  immediately 
into  carbonate  and  permit  of  its  removal  in  solution. 


258       THE   PHYSICAL   MANIFESTATIONS   OF   WEATHERING 

Allusion  has  been  already  made  to  the  fact  that  oxidation 
or  other  chemical  action,  with  the  possible  exception  of  hydra- 
tion,  practically  ceases  below  the  permanent  water  level.  Hunt 
and  Le  Conte  have  both  called  attention  to  the  fact  that  the 
hornblendic  and  feldspathic  rock  fragments  occurring  in  the 
Pliocene  auriferous  gravels  of  California  are  firm  and  intact  in 
those  portions  below  the  drainage  level  (the  blue  gravel  layer), 
but  more  or  less  completely  oxidized,  kaolinized,  and  otherwise 
altered  in  the  red  or  upper  gravel. 

Van  den  Broeck  has  called  attention l  to  the  possibility  that 
the  so-called  red  and  gray  diluvium  of  the  Quaternary  deposits 
near  Paris  may  be  but  portions  of  one  and  the  same  geological 
body,  the  "  diluvium  rouge "  being  but  an  upper  member  of 
the  "  diluvium  gres"  oxidized  and  impoverished  in  lime  by  the 
action  of  meteoric  waters. 

The  same  feature  is  noticeable  in  many  of  our  quarries  for 
building  stone,  as  those  in  the  Berea  sandstones  of  Ohio. 
These  below  the  drainage  level,  are  of  a  gray  or  blue-gray 
color,  while  above,  where  they  have  been  subjected  to  the 
oxidizing  influence  of  meteoric  waters,  they  are  buff.  The 
Jurassic  oolites  of  England,  are  blue-gray  at  some  depths  below 
the  surface,  but  white  above. 

In  cases  where  natural  joint  blocks  are  exposed  to  the  perco- 
lation of  meteoric  waters,  the  weathering  may  for  a  time  mani- 
fest itself  only  in  differential  oxidation  and  zonal  segregation 
of  the  iron  whereby  are  produced  concentric  bands  of  varying 
hues.  Figure  3,  PI.  20,  is  a  slab  from  a  natural  joint  block  of 
argillite  in  the  collections  of  the  National  Museum,  in  which 
the  bands,  due  to  this  cause,  vary  from  yellow-brown,  drab,  to 
ochreous  yellow  and  red,  while  the  rock  as  a  whole  still  retains 
its  compact  structure  and  susceptibility  to  polish,  forming  an 
ornamental  stone  of  no  mean  order.2 

(8)  Relative  Amount  of  Material  removed  in  Solution.  — 
Among  siliceous  rocks,  chemical  action  proceeds  but  slowly, 
and  the  amount  of  material  actually  removed  in  solution  is 
rarely  over  50  %,  and  may  be  so  small  that,  as  the  writer  has 
shown,3  the  residue  in  extreme  cases  occupies  some  80  %  more 
space  than  the  rock  from  whence  it  was  derived.  Carbonate 

1  Bull.  Soc.  Geologique  de  France,  5,  1876-77,  p.  298. 

2  Stones  for  Building  and  Decoration,  p.  169. 

8  Bull.  Geol.  Soc.  of  America,  Vol.  VI,  1895,  pp.  321-332. 


PLATE    20 


FIG.  1.  Weathered  boulder  of  Oriskany  sandstone. 

FIG.  2.  Concentric  weathering  in  diabase. 

FIG.  3.  Zonal  structure  in  weathered  argillite. 

FIG.  4.  Weathered  sandstone,  showing-  induration  along  joint  planes. 


INCIDENTAL   SURFACE   CONTOURS  -      259 

of  lime,  the  essential  constituent  of  ordinary  limestone,  is, 
however,  as  has  been  observed,  soluble  in  the  carbonated  water 
of  rainfalls,  and,  in  time,  may  undergo  complete  removal, 
leaving  but  the  insoluble  impurities  behind.  This  is,  indeed, 
the  almost  universal  history  of  limestone  soils.  They  are  not 
infrequently  so  siliceous  or  ferruginous  as  to  be  quite  barren 
and  of  a  nature  to  be  benefited  by  the  application  of  lime  as  a 
manure. 

Throughout  the  areas  occupied  by  the  Trenton  limestones,  in 
Maryland,  nearly  every  farm  has,  in  years  past,  had  its  quarry 
and  lime-kiln  where  the  stone  was  fitted  for  supplying  lime 
once  more  to  soils  from  which  it  had  been  so  thoroughly  leached 
as  to  render  them  lean  and  poor.  It  is  almost  wholly  to  this 
solvent  action  that  is  due  the  formation  of  the  multitudinous 
caverns,  large  and  small,  of  the  limestone  regions.  Even  where 
caverns  are  not  apparent,  the  corrosive  action  is  evident  to  the 
practised  eye.  In  the  quarry  regions  of  Tennessee  surface 
blocks  of  limestone  are  often  grooved  to  a  depth  of  an  inch  or 
more  with  wonderful  sharpness,  simply  from  the  water  of  rain- 
falls with  its  acids  absorbed  from  the  atmosphere  and  surface 
soils,  while  in  the  quarry  bed  the  stone  is  found  no  longer  in 
continuous  layers,  but  in  disconnected  boulder-like  masses. 
(Figs.  3  and  2,  Pis.  16  and  21.)  In  such  cases  casual 
examinations  give  very  little  clew  to  the  rapidity  of  the  de- 
struction going  steadily  on,  since  all  is  removed  in  solution 
excepting  the  comparatively  small  amount  of  insoluble  matter 
(usually  clay  or  silica)  existing  as  an  impurity. 

(9)  Incidental  Surface  Contours.  —  In  limestone  regions  the 
solvent  action  of  water  has  frequently  gone  on  so  extensively 
as  to  leave  its  imprint  upon  the  topographic  features  of  the 
landscape.  The  drainage  is  no  longer  wholly  superficial,  but 
by  subterranean  streams  sinking  entirely  into  the  ground  to 
reappear  again  at  lower  levels,  it  may  be  miles  away,  having 
traversed  the  intervening  distance  in  some  of  the  numerous 
passages  (fissures  enlarged  by  solution)  with  which  the  rocks 
abound.  Entire  landscapes  are  undulating  through  the  abun- 
dance of  sink-holes  —  shallow  depressions  down  through  which 
the  water  has  percolated  and  escaped  into  the  underground 
passages. 

The  writer  recalls  a  beautiful  illustration  of  this  nature  seen 
in  the  limestone  regions  of  southern  Indiana,  some  years  ago. 


260     '  THE  PHYSICAL   MANIFESTATIONS   OF  WEATHERING 

The  season  was  that  of  the  wheat  harvest.  On  every  side,  far  as 
the  eye  could  reach,  were  undulating  fields  of  waving  grain,  of 
that  charming  golden  hue  of  which  poets  sing,  with  intervening 
patches  of  woodland.  From  every  farm  was  heard  the  click  of 
the  reaper,  and  from  every  fence  the  whistle  of  'the  "  Bob 
White."  Owing  to  the  fact  that  the  ridges  between  these  de- 
pressions were  drier  than  the  bottoms,  the  wheat  here  ripened 
earlier,  and  field  after  field  showed  long  reaches  of  saucer- 
shaped  depressions  green  in  the  centre,  with  intervening  ridges 
of  golden  brown,  making,  with  that  charming  hazy  atmosphere, 
a  picture  long  to  be  remembered.  Through  accident  or  design, 
the  opening  in  the  bottom  of  these  sink-holes  sometimes  becomes 
closed,  giving  rise  thus  to  temporary  pools,  or  ponds,  as  shown 
in  the  accompanying  plate.  It  is  this  same  action  that  has 
given  rise  to  the  so-called  "  saiidpipes "  of  the  English  geolo- 
gists. These  are  slender  funnel-  or  tube-shaped  cavities  found 
in  chalk  and  calcareous  sandstone,  sometimes  filled  with  drift 
gravels,  sands,  brick-earths,  or  again  with  fragmental  materials 
fallen  into  them  from  the  overlying  beds  as  the  support  beneath 
was  gradually  removed.  In  all  these  cases  it  is  assumed  that 
direction  was  given  the  percolating  water  by  pre-existing  fissures 
or  lines  of  weakness.1  (Fig.  1,  PI.  21.) 

In  regions  underlaid  by  massive  siliceous  crystalline  rocks, 
and  where  mechanical  erosion  is  reduced  to  a  minimum,  land- 
scapes are  softly  undulating,  with  few  abrupt  escarpments  or 
precipitous  ledges,  owing  to  the  uniform  rotting  away  of  the 
materials,  and  the  gradual  accumulation  of  the  debris.  It  is  to 
this  form  of  weathering  that  is  due  the  beautiful  rolling  hills 
of  southwestern  Maryland.  The  prevailing  rock  is  granite  or 
gneiss.  Decomposition  follows  out  each  line  of  weakness. 
Streams  erode  through  the  softened  material  down  to  hard 
bed-rock,  while  the  relatively  large  proportion  of  insoluble 
debris  is  left  to  accumulate  on  the  gentle  slopes  which  form 
such  an  enchanting  feature  of  these  landscapes. 

In  regions  of  gneissic  or  granitoid  rocks  traversed  by  large 
veins  of  quartz,  as  in  the  northwestern  part  of  the  District  of 
Columbia,  the  superior  resisting  power  of  the  quartz  causes  it 
to  stand  out  in  relief  from  the  gradually  dwindling  rock  masses 
on  either  hand,  giving  rise  thus  to  prominent  knolls,  or  ridges, 

1  See  Prestwich's  paper,  Quarterly  Journal  Geological  Society  of  London, 
1855,  p.  62. 


INCIDENTAL   SURFACE   CONTOURS  261 

the  occasion  for  which  is  a  mystery  until  we  come  to  examine 
their  foundation  materials.  Belt,  in  describing  the  auriferous 
quartz  lodes  at  San  Domingo,1  states  that  the  prevailing  trend 
of  the  main  ranges  is  nearly  east  and  west,  and  is  probably  due 
to  the  direction  of  the  outcrops  of  the  lodes  which  have  resisted 
the  action  of  the  elements  better  than  the  soft  dolerites. 

So  striking  a  feature  of  the  landscape  as  the  Devil's  Tower 
or  Bear  Lodge  on  Little  Sun  Dance  River,  Wyoming,  is  due  to 
the  weathering  away  and  erosion  of  sedimentary  beds  from 
around  a  dense  crystalline  core  or  plug  of  eruptive  rock  in- 
truded into  them  in  some  past  period  of  volcanic  activity. 
Through  its  greater  powers  of  resistance,  this  still  stands, 
towering  over  1000  feet  above  the  level  of  the  river,  though  in 
time  this,  too,  must  go.  Quite  similar  forms  have  resulted, 
within  a  comparatively  brief  geological  period  through  the 
erosion  of  tufaceous  cones  from  around  the  compact,  crystalline 
plug  of  lava  which  solidified  within  the  crater  when  volcanic 
activity  ceased.  Beautiful  examples  of  these  are  to  be  seen  in 
Arizona  and  New  Mexico,  where  they  are  known  as  "  volcanic 
necks."  The  formation  of  bosses  through  the  influence  of 
joint  planes  has  been  described  elsewhere  (p.  244). 

In  regions  abounding  in  intrusive  olivine  or  pyroxene  rocks 
which  have  undergone  alteration  into  serpentine  and  talc  or 
"soapstone,"  one  frequently  finds  these  materials  forming  the 
main  mass  of  the  hills,  while  the  valleys  are  carved  out  of  the 
softer,  more  readily  decomposed  granite,  or  whatever  the  country 
rocks  may  be.  The  same  feature  is  prominently  developed  in 
the  slate  regions  of  Harford  County,  Maryland,  where  the  slate 
is  the  more  enduring  rock,  and  forms  steep  ridges,  flanked  by 
valleys,  carved  out  from  less  resisting  materials.  Regions  of 
trappean  dikes  in  siliceous  schists  or  gneisses,  particularly 
along  sea-shores  where  swept  by  incoming  tides,  are  often 
characterized  by  narrow,  straight-walled  chasms,  or  canons  due 
to  the  weathering  out  of  the  basic  rocks,  while  the  more  refrac- 
tory schists  on  either  hand  remain. 

In  cases  where  trappean  dikes  have  cut  through  friable  sand- 
stones, they  have  in  some  instances  so  indurated  these  rocks 
along  either  contact  as  to  cause  them  to  be  more  durable  than 
the  original  rock  or  than  even  the  trappean  rock  itself.  There 
may  thus  arise  long  parallel  ridges  of  indurated  sandstone  sepa- 
1  The  Naturalist  in  Nicaragua. 


262       THE   PHYSICAL   MANIFESTATIONS   OF   WEATHERING 

rated  by  an  intervening  depression  due  to  the  weathering  out 
of  the  dike  material. 

In  regions  where  climatic  conditions  or  the  nature  of  the 
rock  are  more  favorable  to  mechanical  disintegration  than 
chemical  decomposition,  contours  may  be  ragged  in  the  ex- 
treme. Entire  crests  may  be  but  successions  of  jagged  peaks 
and  intervening  narrow  valleys  which  are  gradually  becoming 
choked  up  by  the  debris  fallen  from  the  cliffs  above. 

(10)  Effacement  of  Original  Characteristics  through  Weather- 
ing. —  In  cases  of  extreme  decomposition,  in  place,  the  residual 
products  may  so  slightly  resemble  the  parent  rock  as  to  give 
rise  to  very  conflicting  opinions  concerning  their  origin.  This 
was  for  a  long  time  the  case  with  the  laterite  of  India,  already 
described,  and  the  terra  rossa  of  Europe. 

Dana  describes l  an  interesting  case  of  basaltic  decomposition 
which,  on  account  of  the  peculiar  nature  of  the  residual  product, 
is  worthy  of  mention  here.  He  writes:  "  The  process  of  decom- 
position is  finely  exhibited  on  the  second  cliff  north  of  Kiama 
(Australia)  towards  the  north  end.  At  first  sight,  a  distinct 
argillaceous  deposit  was  supposed  to  overlie  the  columnar  basalt; 
for  it  was  twenty  feet  thick,  and  of  a  whitish  color,  resembling 
a  soft  crumbling  marl,  thus  wholly  unlike  the  basalt,  and  the 
common  results  of  basaltic  decomposition.  Still  it  had  pro- 
ceeded from  the  alteration  of  a  regular  columnar  variety,  having 
a  dull  grayish  blue  color.  The  original  rock  is  exceedingly 
compact,  showing  no  trace  of  crystallization,  excepting  an 
occasional  minute  crystal  of  feldspar ;  and  within  the  reach 
of  the  swell,  it  was  still  compact  and  solid. 

"  The  rock  has  a  concentric  structure,  and  to  this  it  owes  in 
part  its  rapid  decomposition.  The  alteration  commences  be- 
tween the  concentric  layers,  rendering  them  apparent,  although 
not  so  before.  At  first  a  thin  ochreous  line  appears,  arising 
from  iron  ;  either  magnetic  iron  disseminated  in  the  rock,  or 
from  that  of  the  constituent  mineral  augite.  This  ochreous 
color  afterwards  mostly  disappears,  and  the  concentric  coats 
become  separated  by  thin  clayey  layers  of  a  white  color,  more 
or  less  striped  with  ochreous  lines.  In  a  more  advanced  stage 
of  the  process  large  ovoidal  masses  of  basalt  (but  little  changed 
in  appearance  excepting  the  development  of  a  slaty  concentric 
structure)  lie  in  the  cliff  separated  by  a  considerable  thickness 

1  Reports  Wilkes's  Exploring  Expedition,  Geology. 


EFFACEMENT  OF   ORIGINAL   CHARACTERISTICS  263 

of  the  whitish  clayey  layers,  which  are  stained  by  irregular 
ochreous  lines.  At  last  the  centres  of  the  spheroidal  masses 
yield,  and  finally  the  change  is  so  complete  that  the  concentric 
arrangement  is  entirely  lost,  and  a  soft  whitish  or  yellowish- 
white  argillaceous  deposit,  with  few  ochreous  spots  or  lines, 
takes  the  place  of  the  compact  basalt. 

"  In  basalts  of  more  compact  structure  these  changes  take 
place  more  slowly.  The  grayish  blue  basalt  in  the  Illawarra 
range,  near  Broughton's  Head,  when  long  exposed,  is  discolored 
exteriorly  to  a  depth  of  an  inch  and  a  half.  The  colors,  begin- 
ning within,  are  dirt-brown,  grayish  yellow,  ochre-yellow, 
brownish  red ;  and  they  are  evidently  dependent  mostly  on 
changes  in  the  condition  of  the  iron  which  the  rock  or  its 
minerals  contain. 

"  When  the  rock  includes  much  chrysolite,  the  results  of 
decomposition  in  some  instances  give  a  fissile  or  micaceous 
appearance  to  the  rock.  At  Prospect  Hill,  five  miles  west  of 
Paramatta,  this  change  is  in  progress.  The  rock  is  a  black 
ferruginous  basalt  of  homogeneous  aspect,  breaking  with  a 
smooth  fracture  and  no  appearance  of  crystallization.  It  con- 
tains chrysolite  ;  but  the  grains  are  small  and  not  apparent 
except  on  very  close  examination.  .  .  . 

"  Were  we  unable  to  trace  the  transitions,  and  distinguish 
the  columnar  structure  through  the  whole,  we  should  scarcely 
suspect  its  basaltic  origin.  Indeed,  it  was  pointed  out  to  me 
as  an  instance  of  mica  slate  overlying  basalt.  Particles  of 
rusted  mica,  as  they  seemed,  were  distinct,  and  it  much  re- 
sembled a  decomposing  variety  of  that  rock.  On  close  inspec- 
tion and  an  examination  of  the  rock  in  different  stages  of 
change,  it  became  evident  that  the  pseudo-mica  was  nothing 
but  altered  chrysolite,  which  had  rusted  from  partial  decompo- 
sition, and  split  into  thin  cleavage  scales. 

"  The  crystals  of  chrysolite  have  evidently  a  parallel  position 
in  the  rock,  and  hence  the  plane  of  easiest  cleavage  lies  in  the 
same  direction,  or,  as  the  cleavage  shows,  parallel  with  the 
upper  surface,  that  is,  at  right  angles  with  the  vertical  axis  of 
the  columns.  The  passage  from  the  compact  to  the  decomposed 
rock  is,  in  this  case,  unusually  abrupt.  Alteration  takes  place 
(through  the  elimination  of  oxide  of  iron  as  before  suggested) 
slowly  at  the  surface,  which  therefore  chips  off  as  soon  as  de- 
composed and  exposes  a  new  portion.  This  sudden  transition 


264       THE   PHYSICAL   MANIFESTATIONS   OF   WEATHERING 

may,  in  part,  proceed  from  the  absence  of  any  natural  planes  of 
fracture  (which  are  brought  out  when  there  is  a  concentric 
structure),  and  perhaps  in  part  also  from  the  presence  of 
chrysolite.  The  layer  of  pseudo-mica  schist  is  in  some  places 
five  feet  thick  and  has  a  rusty  brownish  color.  Above  it  passes 
into  three  feet  of  earth  of  the  same  origin,  having  a  brownish 
black  color,  and  this  is  covered  again  by  four  feet  of  brownish 
red  soil." 

Such  an  effacement  is  not,  however,  an  invariable  accom- 
paniment of  decomposition,  since  where  the  amount  of  residuary 
material  is  relatively  large,  and  allowed  to  accumulate  in  place, 
the  mass  may  for  a  long  period  retain  its  original  structural  char- 
acteristics. Indeed,  the  original  features  are  sometimes  so  per- 
fectly preserved  that  casual  inspection  alone  quite  fails  to  reveal 
the  havoc  that  has  gone  on.  Every  detail  of  bedding,  jointing, 
or  foliation,  or  even  of  internal  structure,  as  brought  about  by 
the  arrangement  or  size  of  the  individual  particles,  may  be  re- 
tained with  perhaps  only  a  slight  change  of  color  due  to  oxida- 
tion. This  feature  is  often  strikingly  conspicuous  in  the  newer 
railway  cuts  of  the  southern  Appalachian  regions,  particularly 
where  the  country  rock  is  of  the  nature  of  gneisses  or  schists. 
In  the  work  of  grading  the  streets,  in  the  extensions  of  the  city 
of  Washington,  masses  of  strongly  foliated  granites,  so  soft  as 
to  be  readily  removed  with  pick  and  shovel,  would  be  cut 
through,  and  which  yet  showed  every  vein  or  other  structural 
detail  as  plainly  marked  as  in  the  original  rock,  and  it  was 
only  when  by  thrusting  one's  cane  or  other  implement  into  it 
that  its  thoroughly  decomposed  condition  became  apparent. 
Russell  describes1  a  similar  condition  of  affairs  prevailing  in 
the  coarse  Triassic  conglomerate  near  Wadesborough,  North 
Carolina.  This  conglomerate  is  here  composed  of  rounded  and 
angular  pebbles  of  talcose  schist  and  other  crystalline  rocks. 
In  the  fresh  cuts  along  the  line  of  the  North  Carolina  railroad, 
every  detail  of  the  original  rock  is  brought  out  almost  as  sharply 
as  in  the  so-called  "  Potomac  marble  "  phase  of  the  same  forma- 
tions as  used  in  the  Capitol  building  at  Washington.  "On 
examining  more  closely,  however,  one  is  surprised  to  find  that 
it  is  completely  decomposed,  and  that  when  moist  it  can  be  cut 
with  a  pocket  knife  through  pebbles  and  matrix  alike,  as  easily 
as  so  much  potter's  clay.  The  full  depth  of  the  alteration  in  this 

1  Bull.  52,  U.  S.  Geol.  Survey,  1889. 


INCIDENTAL   SIMPLIFICATION  OF   COMPOUNDS  265 

instance  is  not  revealed,  but  it  extends  more  than  30  feet  below 
the  surface  without  change  in  character." 

W.  B.  Potter  described l  the  feldspar  porphyry  of  Iron  Moun- 
tain, Missouri,  as  decomposed  to  the  extent  that  it  can  be  easily 
whittled  away  with  a  penknife  or  scratched  with  the  thumb  nail. 
"  At  the  same  time,"  he  writes,  "  the  original  porphyritic 
structure  of  the  individual  crystals  scattered  through  the  mass 
is  beautifully  preserved,  and  is  even  frequently  more  distinctly 
visible  than  in  the  original  rock,  owing  to  stronger  contrasts  of 
color  in  the  kaolinized  material."  In  many  dense  massive  rocks, 
indeed,  such  features  as  flow  structure  and  inequalities  of  text- 
ure are  frequently  rendered  evident  only  on  weathered  surfaces. 
The  same  is  often  true  of  fossiliferous  limestones,  a  weathered 
surface  revealing  the  presence  of  organic  forms  wholly  imper- 
ceptible on  one  freshly  broken. 

The  crude  kaolin  as  removed  from  the  pits  near  Brandywine 
Summit,  Pennsylvania,  and  at  Hockessin,  Delaware,  still  retains 
.more  or  less  distinctly  the  structure  of  the  original  gneiss  or  con- 
glomerate from  whence  it  was  derived.  The  quartz  granules 
of  the  gneiss  are,  in  these  cases,  almost  invariably  shattered, 
as  though  crushed  by  dynamic  agencies,  and  show  distinctly 
corroded  surfaces,  presumably  caused  by  the  alkaline  carbo- 
nates formed  during  the  kaolinizing  of  the  feldspars.  The 
black  mica  makes  its  former  presence  known  by  rust-colored 
spots  which,  in  those  cases  where  the  mineral  was  sufficiently 
abundant,  have  ruined  the  material  for  the  purposes  of  the 
potter. 

(11)  Simplification  of  Chemical  Compounds,  incidental  to 
Weathering.  —  It  has  been  noted  on  p.  172  that  the  process  of 
weathering  is  but  an  attempt  on  the  part  of  the  elements  in 
their  various  combinations  to  adjust  themselves  to  existing  con- 
ditions. This  adjustment  consists  in  the  formation  of  new  com- 
pounds which  are  characterized  by  a  less  complex  structure  than 
those  first  formed. 

Indeed,  one  of  the  most  striking  features  of  chemical  geology 
is  the  tendency  toward  simplification  in  composition  as  mani- 
fested all  over  the  superficial  portions  of  the  earth.  During 
the  process  of  decomposition  there  is  almost  invariably  a  con- 
stant breaking  down  of  complex  molecules  of  mixed  silicates  of 
alumina,  iron,  lime,  magnesia,  and  the  alkalies,  and  a  recombi- 
1  Jour.  U.  S.  Assoc.  Charcoal  Iron  Workers,  Vol.  VI,  p.  25. 


266       THE   PHYSICAL   MANIFESTATIONS  OF   WEATHERING 

nation  of  their  various  elements  as  simpler  silicates,  carbonates, 
sulphates,  and  oxides. 

(12)  Other  Results  incidental  to  Decomposition  and  Erosion.  — 
That  all  the  minerals  of  a  rock  mass  are  not  equally  acted  upon 
by  atmospheric  agencies  has  been  sufficiently  noted  in  previous 
pages.  The  more  refractory,  freed  by  the  breaking  down  of 
their  host,  remain  to  gradually  accumulate  in  vastly  greater 
proportions  than  they  existed  in  the  original  rock.  If,  in 
addition  to  their  refractory  qualities,  such  possess,  as  is  usually 
the  case,  greater  density,  decomposition  and  erosion  may  act  but 
as  agents  of  concentration,  and  in  such  residues  minerals  like 
xenotime  and  monazite  have  been  found  in  abundance,  although 
occurring  so  sparingly  in  the  fresh  rock  that  their  existence  was 
scarcely  suspected. 

It  is  in  this  manner  that  has  originated  the  gem  sand  of 
Ceylon.  Precious  stones  have  been  found  disseminated  in  lim- 
ited numbers  in  the  granite  converted  into  the  cabook  described 
on  p.  242.  In  weathering,  the  difficultly  decomposable  precious 
stones  have  not  been  attacked,  or  attacked  only  to  a  limited  ex- 
tent. They  have  therefore  retained  their  original  form  and  hard- 
ness. When  in  the  course  of  thousands  of  years  streams  of  water 
have  flowed  over  the  layers  of  cabook,  their  soft,  already  half- 
weathered  constituents  have  been  for  the  most  part  changed  into 
a  fine  mud,  and  as  such  washed  away,  while  the  hard  gems  have 
only  been  inconsiderably  rounded  and  little  diminished  in  size. 
The  current  of  water  therefore  has  not  been  able  to  wash  them 
far  away  from  the  place  where  they  were  originally  embedded 
in  the  rock,  and  we  now  find  them  collected  in  the  gravel  bed, 
resting  for  the  most  part  on  the  fundamental  rock  which  the 
stream  has  left  behind,  and  which  afterwards,  when  the  water 
has  changed  its  course,  has  been  again  covered  by  new  layers  of 
mud,  clay,  and  sand.  It  is  this  gravel  bed  which  the  natives 
call  nellan,  and  from  which  they  chiefly  get  their  treasures  of 
precious  stones.1  The  same  process  in  states  bordering  along 
the  Appalachian  Mountain  system  in  North  America  has  given 
rise  to  auriferous  sands,  as  well  as  to  sands  bearing  monazite, 
zircons,  and  other  valuable  minerals,  which  become  segregated 
merely  through  their  greater  density  and  power  to  resist  decom- 
position. The  stream  tin  ores  of  the  Malayan  Peninsula,  the 

1  Nordenskiold,  Voyage  of  the  Vega.    See  also  Judd,  On  the  Rubies  of  Burma, 
etc.,  Fhilos.  Trans.  Royal  Soc.  of  London,  Vol.  CLXXXVII,  1896,  p.  151. 


PLATE   21 


FIG.  1.   Sink-hole  near  Knoxville,  Tennessee. 

FIG.  2.   Beds  of  marble  corroded  by  meteoric  waters,  Pickens  County,  Georgia 


RESULTS   INCIDENTAL   TO  DECOMPOSITION 


267 


diamond-bearing  gravels  of  Brazil,  and  indeed  placer  deposits  in 
general  are  illustrative  of  this  same  principle.  The  very  soil 
itself,  although  so  indispensable  to  human  existence,  is  but  an 
incidental  and  transitory  phase  of  rock-weathering,  as  has  been 
made  sufficiently  apparent  in  previous  pages.  The  deposits  of 
kaolin  in  western  Pennsylvania  and  northern  Delaware,  as  else- 
where noted,  are  but  decomposed  highly  feldspathic  gneisses 
and  conglomerates,  while  the  phosphate  deposits  of  middle  Ten- 
nessee are  insoluble  residue  left  by  the  leaching  out  of  the  cal- 
cium carbonate  from  phosphatic  limestones.1 

According  to  Russell,2  the  Clinton  iron  ore  of  Alabama  is 
but  the  insoluble  residue  from  ferruginous  Silurian  limestones. 
On  the  immediate  surface  the  ore  is  quite  pure,  containing,  it 
may  be,  but  a  trace  of  lime.  When  followed  downward,  the 
amount  of  lime  is  found  to  gradually  increase,  until  the  ores 
may  become  so  poor  in  iron  as  to  be  valueless.  The  following 
figures  show  this  gradual  increase  in  lime  carbonate,  and  neces- 
sary decrease  in  iron,  from  the  surface  downward.3 

PERCENTAGE  OF  CALCIUM   CARBONATE  IN  CLINTON  IRON  ORE 


DEPTH 

PER  CENT 

DEPTH 

PER  CENT 

Surface    

Trace 

70  feet  below  surface   .     . 

25.61 

10  feet  below  surface    .     .     . 

Trace 

80  feet  below  surface   .     . 

29.92 

20  feet  below  surface    .     .     . 

Trace 

90  feet  below  surface   .     . 

29.89 

30  feet  below  surface    .     .     . 

Trace 

100  feet  below  surface   .     . 

23.37 

40  feet  below  surface    .     .     . 

21.06 

110  feet  below  surface    .     . 

28.82 

50  feet  below  surface    .    .    . 

23.90 

120  feet  below  surface   .     . 

21.32 

60  feet  below  surface    .     .     . 

27.01 

130  feet  below  surface   .     . 

30.55 

William  Whitaker  in  1864 4  noted  the  decomposition  of  the 
English  chalk  beds,  in  Middlesex,  and  the  gradual  accumulation 
of  a  stiff  brown-red  residual  clay  interspersed  with  many  flint 
nodules.  It  is  by  this  same  leaching  action  on  aluminous  lime- 
stones that  is  formed  the  so-called  "  rottenstone"  so  commonly 
used  in  polishing  brasses  and  other  metals. 

1  J.  M.  Safford,  American  Geologist,  October,  1896,  p.  261. 

2  Op.  cit.,  p.  22. 

8  Trans.  Am.  Ins.  of  Mining  Engineers,  Vol.  XV,  1886,  p.  189. 
4  Mem.  Geological  Society  of  Great  Britain,  1864,  p.  64. 


THE   WEATHERING  OP  ROCKS  (Continued} 
IV.   TIME    CONSIDERATIONS 

Concerning  the  rate  of  decomposition  of  rocks  of  various 
kinds,  only  very  general  rules  can  be  laid  down,  since  much 
depends  upon  climatic  conditions  and  the  position  of  rock 
masses  relative  to  the  action  of  frost,  moisture,  and  the  various 
growing  organisms. 

(1)  Rate  of  Weathering  influenced  by  Texture.  —  From  the 
study  of  building  materials  it  has  become  apparent  that  a 
coarsely  crystalline  rock  will,  all  other  conditions  being  the 
same,  disintegrate  more  rapidly  than  one  of  finer  grain.  This 
is  doubtless  owing  in  part  to  expansion  and  contraction  from 
ordinary  temperature  variations,  which  act  the  more  energeti- 
cally the  larger  the  crystalline  particles.1 

It  has  already  been  remarked  (ante.  p.  44)  that  crystalline 
rocks  have  a  greater  density  than  do  glassy  forms  of  the  same 
chemical  composition.  This  indicates  a  contraction  during  the 
processes  of  crystallization,  which  manifests  itself,  according  to 
at  least  one  authority,  in  the  development  of  minute  interspaces 
between  the  individual  crystals.  The  coarser  the  crystalliza- 
tion, then,  the  greater  the  amount  of  interstitial  space,  and 
hence  the  greater  the  absorptive  power. 

These  coarser  rocks,  owing  to  their  tendency  to  undergo  a 
mechanical  disintegration,  or  disaggregation,  may  also  yield  to 

1  The  coefficient  of  cubical  expansion  for  several  of  the  more  common  rock- 
forming  minerals  has  been  determined  as  follows :  — 

Quartz.     .     . 0.0000360  Tourmaline 0.000022 

Orthoclase     .     .     .     .     .     .  0.0000170  Garnet 0.000025 

Hornblende 0.0000284  Calcite 0.000020 

Beryl 0.0000010  Dolomite 0.000035 

The  strain  brought  to  bear  upon  a  mass  of  rock  through  the  unequal  rate  of 
expansion  of  its  various  constituents  is  further  complicated  through  the  unequal 
expansion  of  the  individual  minerals  along  the  direction  of  their  various  axes. 
Thus  quartz  gives  a  coefficient  of  0.00000769  parallel  to  the  major  axis,  and  of 
0.000001385  at  right  angles  thereto.  Adularia  gives  0.0000156,  0.000000659,  and 
0.00000294  for  its  three  axes,  and  hornblende  0.0000081,  0.00000084,  and  0.0000095 
(Stones  for  Building  and  Decoration,  p.  419). 

268 


RATE   OF   WEATHERING 


269 


the  decomposing  agencies  more  readily  than  those  of  finer 
grain,  though  from  the  fact  that  they  first  fall  away  to  coarse 
sand,  whereby  the  rock- 
like  character  is  lost,  one 
might,  on  casual  inspec- 
tion, be  led  to  the  oppo- 
site conclusion.  It  need 
scarcely  be  said  that, 
among  rocks  having  the 
same  composition,  wheth- 
er fragmental  or  crystal- 
line, those  of  a  granular 
structure  will  undergo 
disintegration  more 
quickly  than  will  those 
in  which  the  individual 
minerals  are  closely  com- 
pacted or  interknit,  as  in 
many  quartzites  or  dia- 
bases. 

(2)  Rate  of  Weather- 
ing influenced  by  Compo- 
sition. —  Among  rocks  of 
the  same  structure  as  re- 
gards crystallization  and 
size  of  particles,  the  basic 
varieties,  such  as  the  dia- 
bases and  gabbros,  as  a 
rule  succumb  more  read- 
ily than  do  the  more  acid 
varieties  like  the  gran- 
ites. This  for  the  reason 
that  the  iron-magnesian 
as  well  as  the  soda-lime 
minerals  are  more  sus- 


FIG.  20. 


FIG.  21. 


Microstructure  of  sandstone  (Fig.  20),  showing 
relatively  large  amount  of  interstitial  space 
and  absorptive  power,  and  (Fig.  21)  of  dia- 
base, with  relatively  little. 


ceptible  than  are  the  pot- 
ash silicates  and  other 
essential  constituents  of 
the  rocks  of  the  granitic  group.  It  is  possible  also  that  these 
dark  colors  cause  them  to  become  more  highly  heated,  where 
exposed  to  direct  sunlight,  and  hence  subject  to  mechanical  dis- 


270  TIME   CONSIDERATIONS 

integration.  The  fact  that  many  of  our  trappean  rocks,  as  seen 
in  dikes  cutting  other  rocks,  do  not  in  all  cases  succumb  with 
greater  comparative  rapidity  is  due  to  their  very  compact  struct- 
ure, whereby  percolating  waters  are  so  largely  excluded. 

(3)  Rate  of  Weathering  influenced  by  Humidity.  —  The  ra- 
pidity of  rock  weathering  and  soil  formation  is,  even  among 
rocks  of  the   same  nature,  widely  variable,  being  dependent 
upon  climatic  conditions  of  any  particular  locality.    In  the  arid 
regions  north  of  Flagstaff,  Arizona,  are  wide  areas  of  country 
covered  with  coal-black  lapilli  ejected  from  volcanoes  whose 
craters  are  now  occupied  by  growing  pines   upwards   of   two 
feet  in  diameter.    Yet  these  fields  are,  with  the  exception  of  the 
pines,  as  bare  of  vegetation  as  though  but  yesterday  scorched 
by  fire.    The  fine  lapilli,  resembling  nothing  more  than  crushed 
coke,  cover  everywhere  the  undulating  plains,  greedily  absorb- 
ing the  moisture  from  melting  snows  and  scanty  rainfalls,  but 
undergoing  no  appreciable  decomposition  and  affording  foot- 
hold  for  only  a   few  desert   shrubs  and   grasses.     Yet   in   a 
moister  clime,  and  one  more  adapted  for  luxuriant  vegetation, 
we  might  expect  that  these  lapilli  should  long  ago  have  suc- 
cumbed and  given  fairly  fertile  soils. 

(4)  Rate  of  Weathering  influenced  by  Position.  — Among  the 
siliceous  crystalline  rocks  superficial  disintegration  is  undoubt- 
edly greatly  aided  by  temperature  variations,  which,  by  render- 
ing the  rocks  porous,  facilitate  chemical  decomposition.      Such 
action  must,  however,  be  merely  superficial,  and  at  considerable 
depths  below  the  surface  the  change  must  be  purely  chemical. 
The  chief  conditions  favoring  chemical  action  are  those  of  con- 
tinual percolation  by  waters  carrying  the  organic  acids  already 
described.     It  naturally  follows,  therefore,  that  a  purely  chemi- 
cal decay  will  progress  more  rapidly  where  the  rock  mass  is 
covered  by  such  a  layer  of  vegetable  soil  as  shall  give  rise  to 
the  decomposing  solutions.     Hence,  that  such  an  accumulation 
having  begun,  decomposition  will  keep  on  at  an  ever-increasing 
rate  to  a  depth  concerning  which  we  have  at  present  no  data 
for  calculation.     It  must  not  be  too  hastily  assumed  from  this 
that  rocks  thus  protected  do  in  reality  break  down  more  rapidly 
than  those  exposed  on  bare  hillsides,  since  here,  where  physical 
causes  predominate,  the  loosened  particles  are  removed  as  fast 
as  formed,  and,  besides  leaving  no  measure  of  the  destruction 
going  steadily  on,  new  surfaces  for  attack  are  being  continually 


RELATIVE   RAPIDITY   OF   WEATHERING  271 

exposed.  Moreover,  in  assuming  that  rocks  decay  rapidly  where 
covered  by  vegetation,  we  must  not  overlook  the  fact  that  the 
character  of  the  overlying  soil  may  be  such  as  to  be  protective 
rather  than  otherwise.  Thus  in  glaciated  regions  it  is  a  well- 
known  fact  that  the  striae  on  rock  surfaces  are  found  best  pre- 
served where  they  have  been  protected  from  heat  and  frost 
by  a  mantle  of  drift,  or  the  compact  turf  so  characteristic  of 
the  Northern  states.  (See  further  under  Influence  of  Forests, 
p.  280.) 

(5)  Relative  Rapidity  of  Weathering  among  Eruptive  and 
Sedimentary  Rocks.  —  As  to  the  relative  rapidity  of  chemical 
decomposition  among  eruptive  and  sedimentary  rocks,  there 
can — with  the  exception  of  the  calcareous  varieties  —  be  no 
question,  the  eruptives  being  far  the  more  susceptible.  This 
for  reasons  which  will  be  at  once  apparent  when  we  consider 
their  origin.  The  eruptive  rocks  result  from  the  comparatively 
sudden  cooling  of  magmas  originating  far  below  the  action  of 
atmospheric  agencies,  and  which  are  pushed  up  and  allowed  to 
solidify  under  conditions  which  are  not  at  all  conducive  to  chemi- 
cal equilibrium.  They  are  compounds  of  elements  which  have 
combined  according  to  the  conditions  under  which  they  tempo- 
rarily existed,  but  which  undergo  continual  changes  as  they 
become  exposed  by  erosion  and  other  causes.  They  become,  in 
short,  out  of  harmony  with  their  surroundings,  and  there  are  at 
once  set  up  a  series  of  physical  and  chemical  changes  such  as 
shall  result  in  products  more  in  harmony  with  existing  condi- 
tions, and  hence  more  stable.  These  changes,  briefly  put,  are 
those  involved  in  the  weathering  processes  we  have  described. 
Indeed,  we  may  well  say  that  rock  weathering  and  all  the  seem- 
ingly endless  processes  of  rock  decay  and  rock  consolidation 
are  but  stages  in  the  continual  efforts  being  made  by  these  inor- 
ganic particles  to  adjust  themselves  to  existing  conditions.  But 
the  sedimentary  rocks  (exclusive  of  the  calcareous  varieties)  are 
themselves  the  actual  products  of  these  adjustments.  The  con- 
glomerates, sandstones,  shales,  and  argillites  are  but  the  detri- 
tal  remains  of  eruptive  rocks  which  under  the  various  weathering 
influences  have  become  disintegrated  and  decomposed,  their  more 
soluble  constituents  quite  or  in  part  removed,  and  the  residues 
laid  down  and  consolidated  under  conditions  such  as  to-day 
exist  upon  or  near  the  surface  of  the  earth.  They  have,  it  is 
true,  been  laid  down  under  water;  they  are  subaqueous,  but 


272  TIME   CONSIDERATIONS 

their  decomposition  and  disintegration  was  subaerial.  Hence, 
when  elevated  above  the  ocean's  level  to  become  a  part  of  the 
dry  land,  they  are  for  the  most  part  comparatively  stable,  sub- 
ject to  only  such  chemical  changes  as  oxidation,  and  it  may  be 
dehydration.  All  other  things  being  equal,  then,  those  siliceous 
rocks  which  are  the  product  of  mechanical  sedimentation  will  be 
found  far  less  susceptible  to  the  chemical  action  of  the  atmos- 
phere and  meteoric  waters  than  are  the  eruptives.  While  they 
may  undergo  a  transformation  into  soils,  it  is  mainly  through 
the  disintegrating  effects  of  heat  and  frost.  Sedentary  soils 
resulting  from  such  disintegration  resemble,  therefore,  their 
parent  rock  more  than  those  of  any  other  class. 

Turning  now  to  calcareous  rocks,  we  shall  find  a  quite  differ- 
ent state  of  affairs  prevailing,  owing  to  the  different  chemical 
nature  of  the  material  and  its  ready  solubility.  These  rocks 
represent,  in  fact,  the  soluble  portions  of  the  eruptive  rocks 
which  have  been  leached  out  during  the  process  of  decomposi- 
tion. They  are  themselves  solution  products,  although  their 
immediate  deposition  may  have  been  brought  about  through 
mechanical  agencies,  as  in  the  laying  down  of  beds  of  shell 
marl  upon  a  sea-bottom.  The  lime  leached  out  of  terrestrial 
rocks  is  carried  in  solution  into  the  sea,  where,  taken  up  by 
molluscs  and  corals  as  a  carbonate,  it  becomes  precipitated  to 
the  bottom  on  their  death,  and  may  reappear  as  a  limestone,  or, 
if  mixed  with  sufficient  quantities  of  other  constituents,  as  a 
marl,  calcareous  sandstone,  or  shale.  Such  on  their  re-elevation 
are  still  subject  to  chemical  change,  owing  to  the  ready  solu- 
bility of  lime  carbonate  in  terrestrial  waters,  and  so  the  endless 
round  begins  once  more.  Reference  has  already  been  made  to 
the  amounts  of  lime  carbonate  that  may  thus  be  annually  re- 
moved from  the  earth's  surface,  but  one  may  add  here,  that, 
according  to  J.  G.  Goodchild,  certain  English  limestones  waste 
away,  superficially,  at  the  rate  of  one  inch  in  300  years.1 

(6)  Time  Limit  of  Decay.  —  We  are  sometimes  enabled  to 
put  a  time  limit  on  the  beginnings  of  decomposition  such  as 
shall  enable  us  to  gain  at  least  a  geological  measure  of  the 
rapidity  of  the  process.  This  is  the  case  with  the  disintegrated 
granite  of  the  District  of  Columbia  described  on  p.  206.  The 
residual  material  is  here  now  overlaid  by  clastic  deposits  of  such 
a  nature  as  to  force  the  conclusion  that  they  were  laid  down  by 
1  Geological  Magazine,  1890,  p.  463. 


TIME   LIMIT   OF  DECAY  273 

water  under  such  conditions  as  would  have  thoroughly  eroded 
away  all  underlying  pre-existing  decomposed  material.  It  is 
therefore  inferred  that  this  decomposition  has  taken  place  since 
the  clastic  material  was  deposited,  or,  since  these  are  of  Creta- 
ceous age,  that  it  has  taken  place  since  the  close  of  Cretaceous 
times.  In  the  same  way,  since  glaciation  must  have  carried 
away  the  pre-existing  disintegrated  matter  from  the  dike  of 
diabase  at  Medford,  leaving  the  surface  smooth  and  hard,  so 
here  it  is  inferred  that  the  decomposition  is  post-glacial.  It  is 
but  rarely  that  the  rate  of  decomposition  of  any  rock  has  been 
sufficiently  rapid  since  the  beginning  of  human  history,  to  be 
of  geological  significance,  though  weathered  surfaces  in  old 
quarries,  or  the  walls  of  old  buildings,  not  infrequently  offer 
abundant  illustration  of  what  we  might  expect,  could  observa- 
tion be  extended  over  whole  geological  periods  instead  of  at 
most  a  few  years.  We  must  not  forget,  however,  that,  in  the 
latter  case,  the  conditions  are  quite  different  from  those  exist- 
ing in  nature,  and  the  rate  of  weathering  may  be  accelerated  or 
retarded,  as  the  case  may  be. 

Stone  implements,  made  by  prehistoric  man,  as  now  found 
in  graves,  or  dug  from  the  soil,  sometimes  show  incipient  signs 
of  decomposition,  as  indicated,  when  broken  across,  by  a  change 
in  color  and  texture  from  without  inward.  Flint  arrow  and 
spear-heads  from  prehistoric  caves  or  mounds  in  Europe, 
England,  or  America,  often  present  on  the  outer  surface  a  thin 
crust  or  patine  of  a  gray  or  white  color  extending  inward,  it 
may  be,  for  the  distance  of  two  or  more  millimeters.  A  grooved 
stone  axe  of  diorite  found  in  eastern  Massachusetts  and  now  in 
the  collections  of  the  National  Museum  at  Washington,1  shows 
concentric  exfoliation  in  every  way  comparable  to  that  on  the 
diabase  boulder  figured  on  PL  20,  extending  inward  to  a  depth 
of  from  three  to  six  millimetres.  It  is  of  course  possible  that 
the  axe  was  made  from  a  boulder,  itself  not  quite  fresh,  but  this 
seems  scarcely  probable,  and  the  inference  is  fair  that  both  the 
patine  and  the  exfoliation  are  due  wholly  to  weathering  sub- 
sequent to  the  manufacture  of  the  implements  on  which  they 
occur. 

Mills  2  regards  the  extreme  condition  of  decomposition  exist- 
ing in  the  Archrean  rocks  of  Brazil  as  having  taken  place  prior 

1  Specimen  No.  172,794,  Archaeological  Series. 

2  American  Geologist,  June,  1889,  p.  345. 


274 


TIME   CONSIDERATIONS 


to  the  deposition  of  the  loess,  that  is,  in  the  long  interval  between 
the  elevation  of  the  Archaean  rocks  and  the  beginning  of  Qua- 
ternary times.  Inasmuch,  however,  as  the  Quaternary  gravels 
and  loess  are  all  readily  permeable  by  water  and  not  of  a  nature 
to  be  themselves  readily  affected,  it  would  seem  possible  that 
at  least  a  portion  of  the  decomposition  might  have  been  brought 
about  since  their  deposition  and,  indeed,  to  be  still  in  progress. 
The  writer  is  informed  by  Mr.  W.  Lindgren  that  the  granitic 
diorites  of  the  Sierra  Nevadas  of  California,  and  which  are  of 


FIG.  22.  — Flint  implement  showing  weathered  surface. 

late  Jurassic  or  early  Cretaceous  age,  are  often  decomposed  and 
disintegrated  to  a  maximum  depth  of  200  feet,  the  extreme 
upper,  more  superficial  portions  being  reduced  to  the  condition 
of  a  red  clay,  while  the  lower  are  merely  rendered  soft  and 
friable,  with  little  if  any  change  in  color.  This  disintegration 
has  gone  on  to  such  an  extent  that  where  the  rock  is  traversed, 
as  is  sometimes  the  case,  by  numerous  gold-bearing  quartz  veins, 
the  entire  mass  of  material  is  washed  down  by  water — hydrau- 
licked  —  as  in  the  ordinary  process  of  placer  mining.  The 
Pliocene  andesites  are  also  in  places,  decomposed  to  a  depth  of 


TIME   LIMIT   OF   DECAY  275 

20  feet.  The  region  is  one  of  heavy  annual  precipitation,  but 
the  rainfall  is  limited  almost  wholly  to  the  winter  season. 

Rock  disintegration  and  decomposition,  after  the  manner 
already  described,  has  been  by  no  means  limited  to  the  present 
era,  but  has  been  going  on  since  the  first  land  appeared  above 
the  surface  of  the  primeval  ocean.  The  results  of  the  recent 
decomposition  are  more  apparent,  since  the  derived  materials  are 
still  recognizable  as  rock  debris,  while  that  formed  in  past  ages 
may  have  been  so  changed  by  the  solvent  and  assorting  power 
of  water,  the  chemical  action  of  the  atmosphere,  and  the  general 
agents  of  metamorphism,  as  to  have  quite  lost  its  identity. 

Dr.  R.  Bell,  of  the  Canadian  Geological  Survey,  has  described * 
an  interesting  illustration  of  pre-Palseozoic  decay  in  the  crystal- 
line rocks  north  of  Lake  Huron.  The  red  granite,  where  it  has 
been  protected  from  glacial  action,  is  found  to  be  eaten  into 
hollows  in  the  form  of  round  and  sack-like  pits  and  small 
caverns,  the  last-named  generally  occurring  on  steep  slopes  or 
perpendicular  faces  of  the  rock.  These  pits  are,  in  places,  of 
sufficient  size  to  allow  two  men  to  crouch  within.  The  sack- 
like  ovens,  such  as  are  shown  in  Fig.  23,  are  most  usually  on 
sloping  surfaces.  The  granite  around  these  pits  shows  no  in- 
dications of  decay.  That  they  are  of  pre-Palseozoic  origin  is 
demonstrated  by  the  presence  in  them  of  residual  patches,  in 
situ,  of  the  fossiliferous  Black  River  limestone  and  which  Pro- 
fessor Bell  regards  as  veritable  inliers  of  the  Black  River  forma- 
tion, which  once  filled  all  the  inequalities  and  still  overlies  the 
granite  at  lower  levels,  though  elsewhere  almost  wholly  removed 
by  erosion.  Figure  23,  after  Bell,  shows  diagrammatically  the 
old  granitic  corroded  floor  up  on  which  the  calcareous  sediments 
were  laid  down,  with  pits  still  containing  residual  masses  of  the 
limestone,  and  the  intact  beds  passing  under  the  waters  of  Lake 
Huron  at  the  lower  right. 

Pumpelly,  too,  has  shown  2  that  the  diabase  dike  at  Stamford, 
Massachusetts,  had  undergone  extensive  decomposition  prior 
to  the  deposition  of  the  Cambrian  conglomerates.  Of  equal 
interest  and  still  greater  economic  importance  was  the  sugges- 
tion by  this  same  authority,  subsequently  abundantly  confirmed 
by  W.  B.  Potter,3  that  beds  of  iron  ore  lying  on  the  western 

1  Bull.  Geol.  Soc.  of  America,  Vol.  V,  1894,  pp.  35-37. 

2 Ibid.,  Vol.  II,  1891,  p.  209. 

3  Jour.  U.  S.  Assoc.  Charcoal  Iron  Workers,  Vol.  VI,  p.  23. 


276  TIME   CONSIDEKATIONS 

flank  of  Iron  Mountain,  Missouri,  and  covered  by  Silurian  lime- 
stones, were  true  detrital  deposits  resulting  from  the  pre-Silurian 
breaking  down  of  the  ore-bearing  porphyry  forming  the  mass 
of  the  mountain.  These  and  other 1  illustrations  that  might  be 
given  point  unmistakably  to  the  identity  of  geological  processes 
and  correspondence  in  results  since  the  earliest  times,  even  did 
not  analogy  and  the  thousands  of  feet  of  secondary  rocks  furnish 
us  safe  criteria  upon  which  to  base  our  inferences. 


FIG.  23. 

(7)  Extent  of  Weathering.  —  The  depth  to  which  weather- 
ing has  penetrated  necessarily  varies  greatly.  In  cases  where 
the  detrital  material  is  removed  nearly  or  quite  as  rapidly  as 
formed,  it  may  go  on  indefinitely,  until,  it  may  be,  thousands 
of  feet  of  material  have  melted  away ;  where,  however,  remain- 
ing in  place,  decomposition  must  be  gradually  retarded  until  a 
time  comes  when  it  practically  ceases.  In  the  region  about 
Washington,  District  of  Columbia,  the  writer  has  observed  the 
granitic  rock  so  disintegrated  at  a  depth  of  80  feet  from  the 
present  surface  as  to  be  readily  removed  by  pick  and  shovel. 
Even  greater  depths  have  been  noted  by  writers  on  the  geology 
of  our  own  Southern  states  and  Central  and  South  America. 

!See  also  T.  Sterry  Hunt,  The  Decay  of  Rocks  Geologically  Considered, 
Am.  Jour,  of  Science,  Vol.  XXVI,  1883,  p.  190. 


EXTENT   OF   WEATHERING  277 

Spencer  states1  that  in  the  region  about  Atlanta,  Georgia,  the 
rocks  are  " completely  rotted"  to  a  depth  of  95  feet,  while 
''incipient  decay"  may  reach  to  a  depth  of  300  feet.  W.  B. 
Potter  describes2  the  feldspar  porphyry  of  Iron  Mountain  in 
Missouri  as  decomposed  to  a  visible  extent  as  far  into  the  hill 
as  mining  operations  had  been  carried,  while  to  depths  varying 
from  10  to  80  feet  the  kaolinization  is  complete. 

The  coarse  granite  of  Pike's  Peak,  Colorado,  is  reported  as 
disintegrated  to  a  depth  of  from  20  to  30  feet.  Belt3  describes 
dolerites  in  Nicaragua  as  decomposed,  as  shown  by  deep  cut- 
tings in  mines,  to  a  depth  of  200  feet.  "  Next  the  surface,"  he 
says,  "  they  were  often  as  soft  as  alluvial  clay,  and  might  be 
cut  with  a  spade." 

Derby  describes4  certain  shales  in  Rio  Grande  do  Sul,  Brazil, 
as  decomposed  into  the  condition  of  reddish,  drab,  greenish, 
black,  and  umber-colored  clays  to  the  depth  of  120  metres 
(394  feet). 

W.  H.  Fuiionge  has  described5  the  granite  of  the  Dekaap 
gold  fields,  in  the  Transvaal,  South  Africa,  as  decomposed  to  a 
depth  of  200  feet.  Rain  erosion  has  carved  out  from  this 
decomposed  mass  deep  "dongas,"  as  they  are  locally  called, 
and  which  sometimes  present  most  striking  and  picturesque 
appearances. 

The  apparent  depth  to  which  weathering  has  gone  on  is 
often  greater  among  siliceous  than  calcareous  rocks.  This  is, 
however,  due  merely  to  the  facts  that  (1)  the  siliceous  rocks 
are  composed  largely  of  insoluble  materials,  and  hence  leave  a 
proportionately  large  amount  of  debris,  and  (2)  that  among 
calcareous  rocks  the  change  is  mainly  chemical  and  takes  place 
only  from  the  immediate  surface.  As  a  result  of  this,  residuary 
nodules  of  limestone  may  be  found  perfectly  fresh  and  unal- 
tered at  a  depth  of  but  a  few  millimetres  below  the  surface, 
while  granites  and  allied  rocks  may  show  signs  of  disintegra- 
tion and  incipient  decay  for  many  inches,  or  even  feet. 

Pumpelly  states6  that  in  the  Ozark  Mountains  of  Missouri 
the  secular  dissolving  away  of  limestones  containing  from  2  to 

1  Geol.  Survey  of  Georgia,  1893. 

2  Jour.  U.  S.  Assoc.  Charcoal  Iron  Workers,  Vol.  VI,  p.  25. 

3  The  Naturalist  in  Nicaragua,  p.  86. 

4  Am.  Jour,  of  Science,  February,  1884,  p.  138. 

5  Trans.  Am.  Inst.  of  Mining  Engineers,  Vol.  XVIII,  1890,  p.  337. 

6  Am.  Jour,  of  Science,  1879,  p.  136. 


278  TIME   CONSIDERATIONS 

9  %  of  insoluble  matter  has  left  residual  clays  from  20  to  120 
feet  in  thickness,  indicating  a  removal  of  not  less  than  1200 
vertical  feet  by  solution.  According  to  Whitney,  the  dark, 
reddish  brown,  residual  clays  of  .southern  Wisconsin,  of  an 
average  depth  of  perhaps  10  feet  over  the  entire  area,  repre- 
sent the  insoluble  accumulations  from  the  decomposition  of 
from  350  to  400  vertical  feet  of  dolomite,  limestone  and  cal- 
careous shale. 

(8)  Relative  Rapidity  of  Weathering  in  Warm  and  Cold  Cli- 
mates. —  For  many  years  an  impression  has  prevailed  to  the 
effect  that  rocks  decomposed  more  rapidly  in  warm  and  moist 
than  in  cold  climates.  While,  owing  to  abundance  of  vegeta- 
tion and  other  supposed  favorable  conditions,  a  more  rapid 
decomposition  might  be  expected,  such  has  not  as  yet  been 
proven  to  actually  take  place,  and  indeed  many  facts  tend  to 
prove  the  impression  quite  erroneous.  Lack  of  decomposition 
products  in  high  latitudes  is  not  infrequently  due  to  glaciation 
or  erosion  by  other  means.  Whitney,1  Irving,2  Chamberlain, 
and  Salisbury3  have  shown  the  presence  of  residual  clays  of 
all  thicknesses  up  to  25  feet  in  the  driftless  area  of  Wiscon- 
sin, and  Chamberlain  has  described 4  limited  areas  of  strongly 
decomposed  gneiss  in  the  non-glacial  areas  of  Greenland. 

Moreover,  we  have  no  actual  proof  that  the  action  of  frost 
is,  on  the  whole,  protective,  as  is  stated  by  Branner.5  It  must 
be  remembered  that  frost,  excepting  in  the  extreme  north, 
penetrates  to  but  a  slight  depth,  and  while  it  undoubtedly  puts 
a  temporary  stop  to  chemical  action  on  the  immediate  surface, 
it  remains  yet  to  be  shown  that  the  mechanical  disruption  that 
ensues,  and  as  described  in  previous  pages,  is  not  as  efficacious 
as  would  have  been  the  chemical  agencies  alone,  had  they  been 
permitted  to  continue  their  work.  Through  bringing  about  a 
finely  fissile  or  pulverulent  structure,  whereby  a  vastly  greater 
amount  of  surface  becomes  exposed,  frost  prepares  the  way  for 
chemical  action  at  a  thousand-fold  more  rapid  rate  than  could 
otherwise  have  been  possible.  If,  further,  as  the  writer  has 
elsewhere  at  least  suggested,6  hydration  is  the  most  potent 

1  Rep.  Geol.  Survey  of  Wisconsin,  1861. 

2  Trans.  Wisconsin  Acad.  of  Science,  Vol.  Ill,  1875. 

3  Ann.  Rep.  U.  S.  Geol.  Survey,  1884-85,  p.  254. 

4  Bull.  Geol.  Soc.  of  America,  Vol.  VI,  1895,  p.  218. 

6  Bull.  Geol.  Soc.  of  America,  Vol.  VII,  1896,  p.  282. 
6  Bull.  Geol.  Soc.  of  America,  Vol.  VI,  p.  331. 


RELATIVE    RAPIDITY   OF   WEATHERING  279 

factor  in  rock  disintegration,  the  process  can  go  on  uninter- 
ruptedly below  the  level  of  freezing. 

Professor  H.  P.  Gushing  has  described 1  the  argillites  in  the 
vicinity  of  Glacial  Bay,  Alaska,  as  in  a  condition  of  great  dis- 
integration, wholly  through  the  action  of  frost.  "  Disintegra- 
tion," he  says,  "takes  place  with  amazing  rapidity,  as  shown 
by  the  enormous  piles  of  morainic  matter  furnished  to  the  tribu- 
taries of  Muir  Glacier,  whose  valleys  are  adjoined  by  mountains 
of  argillite,  and  by  the  massive  talus  heaps  that  are  rapidly 
accumulating  at  the  bases  of  other  mountains  made  up  of  the 
same  material."  In  a  private  communication  to  the  present 
writer,  he  further  states  that  the  diabases  of  the  region  are 
fully  as  much  decomposed  as  are  those  in  the  Adirondacks  of 
New  York,  and  that  the  blocks  of  eruptive  rocks  occurring  in 
the  moraines  of  Muir  Glacier  are  far  gone  in  decomposition. 

Mr.  C.  W.  Purrington  has  made  similar  observations,  and 
states  2  that  on  the  south  side  of  Silver  Bow  Basin,  some  three 
miles  west  of  Juneau,  at  an  elevation  of  2000  feet  above  sea- 
level,  he  found  schistose  diorites  disintegrated  over  a  consider- 
able area  to  a  depth  of  20  feet.  The  particular  locality  cited 
was  on  a  mountain  slope,  where  landslides  were  frequent,  and 
other  conditions  prevailed  such  as  would  prevent  the  accumula- 
tion of  the  debris  throughout  a  prolonged  geological  period  or 
to  a  very  great  depth.  There  could  be,  however,  no  doubt  as 
to  the  residuary  character  of  the  material  observed,  and  the 
inference  drawn  was  to  the  effect  that  the  disintegration  had 
taken  place  within  a  comparatively  brief  space  of  time.  G.  E. 
Culver  has  also  described  3  a  diabase  dike  in  Minnehaha  County, 
South  Dakota,  an  arid  region  lying  within  the 'glacial  area,  as 
decomposed  throughout  the  whole  exposures  from  its  upper 
surface  down  to  a  depth  of  20  or  25  feet,  the  limit  of  disinte- 
gration being  the  drainage  level  of  the  region  as  marked  by 
the  bed  of  a  stream  cutting  through  it. 

On  the  other  hand,  Professor  I.  C.  Russell,  who  has  devoted 
much  attention  to  the  subject  of  rock-weathering  in  both  high 
and  low  latitudes,  is  of  the  opinion  that  rock  decay  is  a  direct 
result  of  existing  climatic  conditions.  He  states  further  that 
decay  goes  on  most  rapidly  in  warm  regions  where  there  is  an 

1  Trans.  N.  Y.  Academy  of  Science,  Vol.  XV,  1895. 

2  Personal  Memoranda  to  the  writer. 

3  Wisconsin  Academy  of  Sciences,  Art,  and  Literature,  1886-91,  p.  206. 


280  TIME   CONSIDERATIONS 

abundant  rainfall,  and  is  scarcely  at  all  manifest  in  arid  and 
frigid  regions.1  Professor  Russell's  observations  are  of  more 
than  ordinary  value,  since  he  has  discriminated  between  decay 
and  disintegration,  which  most  writers  have  failed  to  do. 

Relative  to  the  subject  of  rock  degeneration  in  temperate 
regions,  we  have  further  to  consider  the  possible  increased 
amounts  of  atmospheric  gases  brought  down  by  snowfalls,  over 
those  brought  by  rain.  The  siiowflakes,  in  falling,  so  com- 
pletely fill  the  air  as  to  rob  it  of  a  larger  proportion  of  its 
impurities  than  would  a  corresponding  amount  of  precipitation 
in  the  form  of  rain.  Further,  the  snow  in  melting  slowly  away 
affords  the  water  better  facilities  for  soaking  into  the  ground 
than  though  it  was  poured  down  during  the  comparatively  brief 
period  of  a  shower.  How  far  these  agencies  may  go  toward 
counterbalancing  the  effects  of  the  continued  higher  tempera- 
tures of  the  tropics,  we  have  no  means  of  judging.2 

It  is  even  questionable  if  decomposition  has  actually  gone  on 
to  greater  depths  in  regions  covered  by  forests,  as  contended 
by  Hartt 3  and  Belt 4  than  elsewhere.  The  accumulation  of  a 
large  amount  of  organic  matter  is  undoubtedly  favorable  to 
decomposition,  but  the  growing  vegetation  constantly  robs  the 
soil  beneath  of  moisture  and  other  elements  necessary  for  its 
growth,  storing  it  away  in  the  form  of  woody  fibre  or  sending 
it  off  into  the  atmosphere  once  more.  The  amount  of  moisture 
that  a  full-grown  tree  evaporates  daily  through  its  leaves  is 
simply  enormous,  and  is  often  made  conspicuously  apparent 
by  the  dry  knolls  that  may  be  seen  surrounding  isolated  trees 
or  groups  of  trees  in  swampy  areas.  Indeed,  Mr.  R.  L.  Fulton, 
in  discussing  5  the  influence  of  forests  in  the  mountain  regions 
of  the  West,  states  it  as  his  belief  that  the  local  springs  and 
streams  are  "  more  diminished  by  the  water  used  by  the  tree& 
than  by  evaporation  in  their  absence." 

It  has  been  shown  6  that  the  total  amount  of  moisture  returned 

1  Surface  Geology  of  Alaska,  Bull.  Geol.  Soc.  of  America,  Vol.  I,  1890. 

2  There  is  an  old  saying  among  Eastern  farmers  to  the  effect  that  a  late 
spring  snowstorm  is  as  good  as  a  dressing  of  manure.    It  undoubtedly  arose 
from  an  appreciation  by  the  farmers  of  the  fact  that  the  snow  was  more  benefi- 
cial than  rain  for  the  reasons  above  mentioned. 

3  Physical  Geography  and  Geology  of  Brazil, 

4  The  Naturalist  in  Nicaragua,  p.  86. 

5  Science,  April  10,  1896. 

6  See  Bull.  No.  7,  Forestry  Division,  U.  S.  Dept.  of  Agriculture,  1893. 


INFLUENCE   OF  FORESTS 


281 


into  the  atmosphere  from  a  forest  by  transpiration  and  evapora- 
tion from  the  trees  and  underlying  soil,  is  about  75%  of  the 
total  precipitation.  For  other  forms  of  vegetation  it  varies 
between  70  %  and  90  %,  the  forest  as  a  rule  being  surpassed  by 
the  cereals,  while  the  evaporation  from  a  bare  soil  is  but  30  % 
of  the  precipitation.  To  this  should  be  added  the  fact  that 
the  activity  of  evaporation  from  forested  areas  is  continued 
throughout  a  longer  period  of  each  year,  as  a  rule,  than  in 
non-forested,  for  the  simple  reason  that  the  grasses  and  cereals 
early  ripen,  and  practically  cease  to  exhale  altogether.  This 
is  particularly  the  case  in  cultivated  areas  and  prairie  regions. 
Hence,  while  the  daily  evaporation  from  given  areas  might  for 
a  time  be  nearly  equal,  the  annual  amount  is  likely  to  be  great- 
est for  that  which  is  forested. 

Further,  it  has  been  shown  that  only  70  %  as  much  rainfall 
reaches  the  soil  in  the  woods  as  in  the  open  fields,  the  rest 
being  caught  in  the  leaves,  branches,  and  trunks,  whence  it  is 
returned  directly  to  the  atmosphere  by  evaporation.  These 
percentages  naturally  vary  with  the  character  of  the  forest 
growth.  In  this  connection  the  following  table,  showing  the 
measured  amounts  of  water  at  varying  depths  in  a  loamy  soil 
under  forests  of  spruce,  twenty-five,  sixty,  and  one  hundred 
and  twenty  years  old,  and  one  base  of  all  vegetation,  is  instruc- 
tive. It  will  be  observed  that  the  average  amount  is  apprecia- 
bly greater  in  the  bare  soil,  and  that  the  least  amount  is  found 
under  forests  60  years  old,  when  we  may  assume  the  trees  are 
in  their  prime. 

WATEU   CONTENTS   OP  A  LOAMY   SAND;    RESULTS   BY   SEASONS   EXPRESSED  ix 
PERCENTAGES  OF  THE  WEIGHT  OF  THE  SOIL 


SEASON 

SPRUCE 

25  YEAKS  OLD 

60  YEARS  OLD 

16  inch 

32  inch 

Average 

16  inch 

82  inch 

Average 

Winter  (January  and  February)  . 
Spring  (March  to  May)  .... 
Summer  (June  to  August)  .  .  . 
Fall  (September  to  November)  .  . 

20.23 
18.62 
15.10 
16.57 

17.00 
18.02 
16.22 
17.57 

18.61 
18.32 
15.96 
17.07 

18.06 
15.29 
14.42 
13.49 

17.76 
16.28 
17.03 
16.52 

17.91 
15.78 
15.72 
15.00 

282 


TIME   CONSIDERATIONS 


SEASON 

SPRUCE 

NAKED  SOIL 

120  YEARS  OLD 

16  inch 

32  inch 

Average 

16  inch 

32  inch 

Average 

Winter  (January  and  February)    . 
Spring  (March  to  May)      .... 
Summer  (June  to  August)     .     .     . 
Fall  (September  to  November)  .     . 

19.75 
17.47 

17.78 
14.88 

22.44 
20.83 
20.90 
19.46 

21.09 
19.15 
19.97 
17.17 

19.96 
20.66 
19.77 
20.04 

24.73 
20.51 
19.98 
20.20 

22.35 
20.58 
19.97 
20.12 

Other  experiments  have  shown  a  marked  difference  in  the 
distribution  of  the  water  in  the  forest-covered  and  naked  soils, 
in  the  first-named  a  much  larger  proportion  being  held  in  the 
extreme  upper  portion  than  in  that  which  was  unprotected. 
This  is  a  natural  consequence  of  the  absorptive  properties  of 
the  accumulated  humus.  The  following  table,  as  compiled  by 
Fernow l  from  the  work  of  Ebermayer,  illustrates  this  point. 

AVERAGE  OF  WATER  CAPACITY,  EXPRESSED  IN  PERCENTAGES  OF  THE  WEIGHT 

OF  THE  SOIL 


SPRUCB 

DEPTH 

25  Years 
Old 

60  Years 
Old 

120  Years 
Old 

SOIL 

0  to  2  inches 

30  93  % 

29  48  °L 

40  32  % 

22  33  °/ 

6  to  8  inches    

19.19 

18.99 

19.30 

20  62 

12  to  14  inches  

19.10 

16.07 

18  28 

20  54 

19  to  20  inches 

18  40 

16  26 

20  16 

20  14 

30  to  32  inches  . 

17  91 

17  88 

21  11 

20  54 

It  is  obvious  that  it  is  only  that  portion  of  the  water  which 
passes  through  this  superficial  blanket  of  mould  that  can  be 
instrumental  in  promoting  rock  decomposition.  Hence  the 
presence  of  such  a  blanket  may  exert  a  protective,  or  at  least 
conservative,  rather  than  destructive  action.  Further  than  this, 
we  have  to  remember  that  plant  growth  tends  to  reduce  the 
extremes  of  temperature  and,  even  more,  to  diminish  evapora- 

1  Bull.  No.  7,  Forestry  Division,  U.  S.  Dept.  of  Agriculture,  1893. 


WEATHERING   IN   COLD   AND   WARM   CLIMATES  283 

tioii  from  the  immediate  surface.  The  constant  action  of  grav- 
ity and  capillarity  in  pumping  the  water  down  and  up  through 
the  soil  is  therefore  largely  diminished.  Since  it  is  by  temper- 
ature changes  and  water  action  that  decomposition  is  so  largely 
brought  about,  it  is  apparent  that  we  must  not  be  too  hasty  in 
assuming  that  forest  action  is  actually  destructive  ;  it  may  be 
largely  conservative.  It  is  possible  that  the  apparent  amount 
of  decomposition  in  wooded  areas  is  due  to  protection  from  ero- 
sion, and  the  consequent  accumulation  of  the  residuary  material. 
More  facts  are  necessary  before  this  question  can  be  decided. 

(9)  Difference  in  Kind  of  Weathering  in  Cold  and  Warm. 
Climates.  —  That,  however,  there  may  be  a  difference  in  kind 
in  the  degeneration  in  warm  and  cold  climates,  or  at  least  in 
moist  and  dry  climates,  is  possible  and  even  probable.1  In  cold 
and  in  dry  climates  subject  to  extremes  of  temperature,  as  in 
the  arctic  regions  or  in  the  arid  regions  of  lower  latitudes,  the 
weathering  is  at  first  almost  wholly  in  the  nature  of  disintegra- 
tion, a  process  of  disaggregation  whereby  the  rock  is  resolved 
into,  first,  a  gravel  and  ultimately  a  sand  composed  of  the 
isolated  mineral  particles  which  have  suffered  scarcely  at  all 
from  decomposition.  The  writer  has  elsewhere  referred  to  this 
form  of  degeneration  as  manifested  in  the  desert  regions  of  the 
Lower  California!!  peninsula.2  In  a  warm,  moist  climate  chem- 
ical decomposition  may  or  may  not  keep  pace  with  the  disin- 
tegration, according  to  local  conditions,  so  that  the  resultant 
material  may  be  in  the  form  of  an  arkose  sand,  as  in  the  District 
of  Columbia,  or  a  residual  clay,  as  in  the  more  superficial  portions 
of  the  residual  deposits  to  the  southward.  In  certain  cases,  or 
among  certain  classes  of  rocks,  the  decomposition  proceeds  at  so 
rapid  a  rate  that  there  is  scarcely  any  apparent  preliminary  dis- 
integration. Local  circumstances  and  character  of  rock  masses 
being  the  same,  we  are,  however,  apparently  safe  in  assuming 
that  in  warm  and  moist  climates  decomposition  follows  so  closely 
upon  disintegration  as  to  form  the  more  conspicuous  feature  of 
the  phenomenon,  while  in  dry  regions,  or  those  subject  to  ener- 
getic frost  action,  mechanical  processes  prevail  and  disintegra- 
tion exceeds  decomposition. 

1  The  majority  of  writers  have  failed  to  discriminate  between  decomposition 
and  disintegration.    That  there  may  be  a  very  marked  difference,  due  mainly  to 
climatic  conditions,  is  the  point  I  wish  to  emphasize  here. 

2  Bull.  Geol.  Soc.  of  America,  Vol.  V,  1894,  p.  499. 


284  TIME   CONSIDERATIONS 

Accepting  these  facts,  there  is  at  once  suggested  the  idea 
that  the  lithe-logical  nature  of  sedimentary  rocks,  as  well  as 
their  fossil  contents,  may  be  regarded  as  indicative  of  prevalent 
climatic  conditions. 

The  possibility  of  estimating  these  conditions  by  the  char- 
acter of  the  debris  resulting  from  the  degeneration  of  feld- 
spathic  rocks  was  first  suggested  by  the  geologists  of  the  Indian 
Survey,1  the  undecomposed  feldspars  in  the  Panchet  (Mesozoic) 
sandstones  being  regarded  as  indicating  a  recurrence  of  a  cold 
period  during  which  mechanical  forces  preponderated  over  those 
purely  chemical.  The  same  idea  was  subsequently  put  forth, 
quite  independently,  by  the  present  writer.2  That  rocks  in  arid 
regions  do  actually  undergo  less  decomposition  during  the 
weathering  processes  is  shown  not  only  by  the  fresh  character 
of  the  residuary  material.  Judd  has  shown3  that  rivers  like 
the  Nile,  draining  regions  of  great  aridity,  though  in  a  con- 
dition of  high  concentration  from  prolonged  evaporation,  carry, 
in  solution,  smaller  proportional  amounts  of  derived  salts  than 
do  those  of  humid  regions. 

Russell  has  noted  that  in  the  Yukon  River  region  of  Alaska 
disintegration  so  far  exceeds  decomposition  that  the  talus  from 
the  mountains,  composed  of  loose,  angular  masses  of  rock  quite 
free  from  vegetation,  forms  what  he  calls  debris  streams,  which 
actually  creep  slowly  down  the  slopes,  the  movement  taking 
place  principally  in  the  winter  time  and  being  due  apparently 
to  the  slow  settling,  or  creep,  of  deep  snows.  He  states  it  as 
his  opinion  that  the  mountains  of  the  region  have  suffered  more 
through  this  form  of  disintegration  than  have  those  of  Colorado 
or  the  southern  Appalachians,  but  less  than  those  of  the  Great 
Basin  area.  The  range  of  limestone  mountains  along  the  Yukon 
is  pictured  as  presenting  a  crest  of  sharp,  blade-like  crags,  flanked 
by  vast  slopes  of  loose,  angular  stones  on  either  side,  the  rock 
being  everywhere  fresh  and  undecomposed,  but  badly  shattered 
and  fissured. 

(10)  Relative  Amount  of  Material  lost. —  Other  things  being 
equal,  it  is  also  safe  to  infer  that  more  material  has  actually 
been  lost  through  disintegration  and  decomposition  in  moun- 

1  Geol.  of  India,  2d  ed.,  Vol.  I,  p.  201. 

2  Bull.  Geol.  Soc.  of  America,  Vol.  VII,  p.  362. 

3  Report  on  Deposits  of  the  Nile  Delta,  Proc.  Royal  Society  of  London,  Vol. 
XXXIX,  1885. 


PLATE   22 


FIG.  1.   Forest  destroyed  by  wind-blown  sand. 

FIG.  2.   Calcareous  conglomerate  carved  and  polished  by  wind-blown  sand. 

FIG.  3.   Rock  being  undermined  by  wind-blown  sand. 


RELATIVE   AMOUNT  OF   MATERIAL   LOST  285 

tainous  and  hilly  countries  than  from  the  level  plains.  This 
for  the  reasons  that  (1)  through  the  upturning  of  the  beds  there 
were  exposed,  it  may  be,  friable  and  soluble  strata  that  might 
otherwise  have  been  protected,  and  (2)  that  through  the  shat- 
tering incident  to  this  upturning  the  rocks  were  rendered  more 
susceptible  to  the  weathering  forces.  Further,  (3)  the  steeper 
slopes  in  mountain  regions  promote  more  rapid  removal  of  the 
resultant  debris,  whereby  fresh  surfaces  are  continually  exposed, 
such  as  might  otherwise  shortly  become  protected  through  its 
accumulation,  as  above  noted. 


PART    IV 

THE  TRANSPORTATION  AND  RBDBPOSITION  OP 
ROCK   DEBRIS 

IT  rarely  happens  that  more  than  a  comparatively  small  pro- 
portion of  the  products  of  disintegration  and  decomposition  are 
left  to  accumulate  on  the  site  of  the  parent  rock.  In  most 
instances  a  very  considerable  proportion,  in  some  instances  all, 
the  debris  is  removed  immediately,  or  soon  after  its  formation, 
and  deposited  elsewhere.  A  portion  of  this  material  is  removed 
in  solution,  as  has  already  been  described  (ante,  p.  194).  A 
still  larger  portion  is  transported  mechanically,  and  it  is  to  a 
discussion  of  the  method  of  this  transportation  that  a  few  pages 
may  now  be  devoted  with  profit. 

The  chief  agencies  involved  in  this  transportation  are  grav- 
ity, water,  in  either  a  solid  or  liquid  form,  and  the  wind.  Un- 
doubtedly the  major  part  of  the  work  is  done  by  water,  but  as 
the  wind's  action  is  so  frequently  overlooked,  and  as,  moreover, 
the  results  thus  produced  are  of  more  than  ordinary  interest 
from  our  present  standpoint,  it  may  perhaps  be  well  to  dwell 
upon  this  branch  of  the  subject  with  considerable  detail. 

(1)  Action  of  Gravity.  —  Gravity,  especially  when  aided  by 
the  lifting  power  of  frost,  may  locally  exert  no  insignificant 
influence.  The  tremendous  power  of  landslides,  or  avalanches, 
have,  owing  to  their  devastating  effects,  been  impressed  upon 
us  from  the  beginnings  of  written  history.  There  are,  how- 
ever, other  results,  due  to  similar  causes,  but  which,  going  on 
on  an  almost  microscopic  scale,  are  wholly  overlooked  by  the 
ordinary  observer,  and  the  full  meaning  of  which  can  be  dis- 
covered only  when  the  results  of  years  are  taken  into  account. 
Professor  W.  C.  Kerr,  in  1881,  described1  the  manner  in  which 
the  superficial  cap  of  soil  from  the  decomposition  of  micaceous 

1  Am.  Jour,  of  Science,  3d  Series,  Vol.  XXI,  p.  345. 
286 


ACTION   OF   WATER   AND   ICE  287 

and  hornblendic  gneisses  near  Philadelphia  had  crept  down 
the  inclined  surface  on  which  it  rested,  and  the  gradual  attenu- 
ation of  the  bands  of  variously  colored  debris  of  Avhich  it  was 
composed.  This  creeping  process  he  ascribed  wholly  to  the 
expansive  action  of  included  water  passing  into  the  condition 
of  ice,  the  expansion  taking  place  laterally  and  the  material 
being  pushed  down  the  slope  along  the  line  of  least  resistance. 
Mr.  C.  Davidson  has  since  taken  up  the  subject  experimentally 


FIG.  2-L  —  Showing  direction  and  rate  of  motion  of  soil ;  the  arrows  showing,  by 
their  relative  lengths,  the  rate  of  movement  at  various  points,  a,  soil;  b,  bed- 
rock. 

and  shown  that  the  amount  of  the  creeping  could  be  accounted 
for  by  the  ordinary  laws  of  gravity,  the  frost,  by  its  expansion, 
raising  the  individual  particles  a  slight  distance,  and,  on  thaw- 
ing, allowing  them  to  drop  back  again  a  greater  or  less  distance 
down  the  slope,  according  to  the  angle  of  inclination.  Dr. 
Milton  Whitney  has,  however,  shown1  that  there  is  an  almost 
continual  movement  among  soil  particles,  dependent  upon 
meteorological  conditions  quite  aside  from  those  involved  in 
freezing  and  thawing.  The  creeping  appears  therefore  to  be 
but  the  manifestation,  in  mass,  of  the  inclination  of  each  indi- 
vidual particle  to  slide  down  the  slope. 

The  accumulations  of  talus  at  the  foot  of  every  cliff  and  on 
the  slopes  of  hills  and  mountains  are  matters  of  such  every -day 
observation  as  to  need  no  mention  in  detail. 

(2)  The  Action  of  Water  and  Ice.2  —  The  power  of  a  stream 
to  transport  rock  debris  depends  naturally  upon  its  volume 
and  the  rapidity  of  its  current.  This,  on  the  supposition  that 
the  character  of  the  sediment  to  be  transported  remains  the 

1  Some  Physical  Properties  of  Soils,  Bull.  No.  4,  U.  S.  Weather  Bureau,  1892. 

2  Students  are  referred  to  Professor  R.  D.  Salisbury's  article  on   Agencies 
which  transport  Material  on  the  Earth's  Surface,  Journal  of  Geology,  Vol.  Ill, 
1895,  p.  70. 


288      TRANSPORTATION  AND  REDEPOSITION  OF  ROCK  DEBRIS 

same.  According  to  the  calculations  of  Hopkins,  as  quoted  by 
Geikie,1  the  capacity  of  transport  increases  as  the  sixth  power 
of  the  velocity  of  the  current ;  that  is  to  say,  the  motor  power 
is  increased  sixty-four  times,  by  doubling  the  velocity.  The 
following  table  is  taken  from  the  work  quoted  as  showing  the 
power  of  transport  of  river  currents  of  varying  velocities:  — 

INCHES  MILES 

PER  SECOND  PER  HOUR 

3  0.170   :   will  just  move  fine  clay. 

6  0.240   :   will  lift  fine  sand. 

8  0.4545:   will  lift  sand  as  coarse  as  linseed. 

12  0.6819 :   will  sweep  along  fine  gravel. 

24  1.3638  :   will  roll  along  rounded  pebbles  1  inch  in  diameter. 

36  2.045   :  will  sweep  along  slippery,  angular  stones  of  the  size  of  an 
egg. 

There  are,  of  course,  other  factors  that  should  be  taken  into 
consideration,  such  as  the  character  of  a  river  bed,  the  density 
of  the  water,  etc.,  but  which  lack  of  space  prevents  our  touch- 
ing upon  here,  and  which  are,  moreover,  sufficiently  enlarged 
upon  in  other  works. 

The  writer  has  stood  at  the  head  waters  of  the  Missouri,  and 
seen  the  Jefferson,  Madison,  and  Gallatin  rivers  uniting  their 
floods  to  form  one  grand  rushing  stream  of  clear  green  water, 
full  of  trout  and  grayling.  He  has  seen  it  again  at  Mahdan, 
Dakota,  a  sluggish  stream  actually  yellow  with  suspended  silt. 
At  St.  Louis,  one  beholds  it  a  mighty  torrent,  whirling  along 
trunks  and  stumps  of  trees,  twigs,  and  all  manner  of  organic 
debris  and  inorganic  detritus  picked  up  from  its  banks,  or 
washed  in  by  rains  and  tributary  streams,  till,  one  vast  sea  of 
liquid  mud,  it  pours  every  year  into  the  Gulf  of  Mexico  a  mass 
of  sediment  equal  to  812,500,000,000,000  pounds  (7,468,694,400 
cubic  feet),  or  enough  to  cover  a  square  mile  of  territory  to 
a  depth  of  268  feet.  But  only  a  portion  of  the  detritus  car- 
ried by  running  streams  reaches  the  ocean ;  otherwise  we  need 
devote  little  time  here  to  its  consideration.  Nearly  all  streams, 
in  some  part  of  their  courses,  flow  through  level  plains  with  low 
banks  which  are  subject  to  inundation  during  seasons  of  high 
water.  Picture  a  muddy  stream  such  as  is  shown  in  cross-sec- 
tion in  Fig.  25,  and  which  at  ordinary  periods  is  confined 
within  the  narrow  channel  near  the  centre.  In  time  of  freshet, 
however,  the  volume  of  water  is  so  greatly  augmented  as  to 

1  Text-book  of  Geology,  3d  ed. 


ACTION   OF    WATER   AND   ICE  289 

cause  it  to  overflow  its  banks  and  spread  out  over  the  plains  on 
either  hand.  But  110  sooner  does  the  water  leave  the  channel 
than  the  force  of  its  currents  becomes  checked,  its  carrying 
power  lessened,  and  it  therefore  begins  to  deposit  its  load  of 
silt  upon  this  flood  plain,  as  it  is  called,  where  it  remains  to 
permanently  enrich  the  land  when  the  waters  subside.  It  is  to 
such  processes  of  formation  that  we  owe  some  of  the  most  fer- 
tile lands  in  existence,  as  the  valley  of  the  Mississippi,  that  of 
the  Red  River  of  the  North,  the  Nile,  and  scores  of  others  that 
might  be  mentioned  readily  attest.1 


FIG.  25. 

To  the  same  process,  coupled  with  the  accumulation  of 
organic  matter,  we  owe  the  filling  in  and  gradual  extinction  of 
thousands  of  glacial  lakes  throughout  New  England  and  the 
North,  and  the  formation  of  rich,  flat-bottomed  valleys  known 
locally  as  meadows,  swales,  and  bogs. 

Ice  in  the  form  of  glaciers  is  an  efficient  agent  for  transpor- 
tation as  well  as  for  erosion,  as  already  noted.  While  the  work 
being  done  by  existing  glaciers  may  seem  comparatively  insig- 
nificant, that  done  by  the  ice  sheet  of  the  glacial  epoch  was  by 
no  means  so,  and  deserves  a  more  than  passing  notice.  The 
manner  in  which  the  ice  carries  and  deposits  its  load  has  already 
received  attention  in  speaking  of  its  erosive  power,  and  but 
little  more  need  be  said  on  the  subject.  That  material  which 
existed  in  a  loose,  unconsolidated  condition,  on  the  surfaces  on 

1  The  Arkansas  River  is  stated  by  Owen  (Geol.  of  Arkansas,  2d  Rep.,  1860, 
p.  52)  to  be  at  certain  seasons  of  the  year  almost  blood-red  from  the  quantity  of 
suspended  fine  ferruginous  clay  and  saliferous  silt  brought  down  from  the  regions 
of  ferruginous  shales,  which  prevail  in  the  Cherokee  County,  through  which  the 
river  flows.  This  material,  deposited  along  the  banks  and  in  the  eddies  of  still 
water,  produces  the  celebrated  red  buckshot  land.  Material  washed  from  the 
bluffs  of  argillaceous  shell  marl,  near  the  confines  of  Jefferson  and  Pulaski 
counties,  is  deposited  again  farther  down  the  stream  as  a  fine  silt,  imparting, 
like  the  red  silt,  extraordinary  fertilizing  properties  to  the  soil. 


290     TRANSPORTATION  AND  REDEPOSITION  OF  ROCK  DEBRIS 

which  the  glacier  formed,  was  pushed  and  dragged  along  by 
the  onward  movement  of  the  ice,  which  in  extreme  cases  may 
have  exerted  a  pressure  of  200,000  pounds  to  the  square  foot. 
On  the  final  retreat  of  the  glacier,  this  was  left  in  the  form  of  a 
compact  structureless  mass  of  almost  stony  hardness,  commonly 
known  as  till  or  ground  moraine,  Materials  falling  upon  the 
surface  from  greater  heights  were  likewise  transported,  so  long 
as  the  ice  sheet  continued  to  advance,  and  finally  deposited  in 
the  form  of  terminal  or  frontal  moraines. 

Inasmuch  as  the  ice  sheet  was  almost  continually  melting 
upon  its  surface,  it  is  practically  impossible  to  consider  its 
action  wholly  independent  of  that  of  water  also.  Thus, 
streams  resulting  from  such  melting  would  gradually  wear 
channels  in  the  ice,  as  on  the  land.  In  these  channels  would 
accumulate  sand  and  boulders  of  such  size  and  weight  as  to 
resist  the  current,  and  such  accumulations  would,  on  the  final 
melting  of  the  sheet,  be  deposited  on  the  surface  of  the  ground 
in  the  form  of  ridges  known  as  eskers,  or  osars.  Other  forms 
of  water  action  on  the  materials  of  the  ice  sheet,  are  hillocks 
of  stratified  sand  and  gravel  deposited  near  the  terminal  mo- 
raines, and  known  as  kames.  Since  during  the  advancing  of 
this  ice  sheet  existing  rivers  flowing  eastward  must  have  been 
dammed,  we  can  safely  imagine  the  formation  of  large  tempo- 
rary lakes,  on  the  bottom  of  which  would  be  deposited  the 
glacial  silt,  like  the  so-called  loess  of  the  Mississippi  valley. 
Lake  Agassiz,  a  glacial  lake  of  this  type,  is  supposed  to  have 
occupied  an  area  of  more  than  100,000  square  miles  in  north- 
western Minnesota,  northeastern  Dakota,  and  a  considerable 
portion  of  Manitoba.  On  the  bottom  of  this  lake  there  was 
deposited  during  the  comparatively  brief  time  of  its  existence, 
silt  to  a  depth  as  yet  undetermined,  but  known  to  be  at  least 
100  feet.1 

Waters  issuing  from  the  melting  ice  sheet  tend  to  reassert  the 
material  of  the  terminal  moraine,  redepositing  it  in  approxi- 
mately concentric  zones  beyond  its  margin.  These  deposits 
are  naturally  thicker  and  coarser  near  the  moraine  and  thinner 
and  finer  at  increasing  distances.  Their  form  and  mode  of 
occurrence  is  such  as  to  have  suggested  for  them  the  name  of 
glacio-fluvial  aprons,  or  frontal  aprons.  Their  materials  are 
nearly  always  loose  sands  and  gravels,  the  lithological  nature 
1  Ice  Age  in  North  America,  by  G.  F.  Wright,  p.  355. 


ACTION   OF    WATER   AND    ICE  291 

of  the  individual  particles  being  of  course  dependent  upon  that 
of  the  moraines  from  which  they  are  derived. 

The  effects  upon  the  landscapes  of  this  ice  sheet  have  been 
lasting  and  peculiar.  We  may  safely  imagine  that,  before  the 
ice  invasion,  the  surface  was  covered  with  decayed  and  softened 
materials  like  the  residual  soils  of  our  Southern  states,  and 
which  had  been  cut  up  into  valleys  and  intervening  ridges  by 
the  stream  of  that  time.  The  ice  sheet  stripped  from  these 
surfaces  their  mantle  of  decomposed  materials,  and  in  addi- 
tion cut,  in  many  cases,  into  the  fresh  rock,  actually  planing 
the  entire  country  so  deeply  that  in  most  cases  the  preglacial 
surface  is  no  longer  recognizable.  The  hills  were  thus  lowered 
and  the  valleys  in  some  cases  deepened  or  again  filled  by  sand 
and  gravel.  Since  a  protruding  rock  mass  would,  from  neces- 
sity, be  most  eroded  on  the  side  from  whence  the  ice  sheet 
approached,  and  since,  moreover,  such  would  serve  to  catch 
and  hold  back  a  part  of  the  loose  earth  and  stony  matter 
brought  from  the  north,  a  peculiar  feature  in  the  topography 
of  glaciated  hills  has  been  brought  about  as  shown  in  Fig.  2, 
PI.  25. 

The  direction  taken  by  this  drift  material  was  quite  variable. 
It  was,  as  a  rule,  from  the  north  toward  the  south,  with  many 
minor  deflections.  Boulders  of  Laurentian  rocks  north  of  Lake 
Huron  are  abundant  in  the  drift  about  Oberlin,  Ohio,  and  even 
further  south.  Boulders  of  native  copper  from  the  Lake  Supe- 
rior region  are  found  even  as  far  south  as  Kankakee,  Illinois, 
and  a  large  boulder  of  a  peculiar  conglomerate  known  in  place 
only  near  Ontario,  has  been  found  a  few  miles  south  of  the 
Ohio  River  in  Kentucky.  Dawson  states  "  that  boulders  from 
the  Laurentian  axis  of  the  continent,  which  stretches  from 
Lake  Superior  northward  to  the  west  of  Hudson  Bay,  have 
been  transported  westward  a  distance  of  700  miles,  and  left 
upon  the  flanks  of  the  Rocky  Mountains  at  an  elevation  of 
something  over  4000  feet."1 

All  over  the  states  once  occupied  by  this  ice  sheet  the  ma- 
terial originating  from  the  decomposition  of  rocks  in  situ,  or 
deposited  on  alluvial  plains,  was,  with  a  few  minor  exceptions, 
carried  away  to  the  southward  and  in  part  dumped  into  the 
Atlantic,  while  its  place  was  supplied  by  mongrel  hordes  from 
the  north.  In  process  of  digging  for  the  foundations  of  the 
1  Ice  Age  in  North  America,  p.  171. 


292     TRANSPORTATION  AND  REDEPOSITION  OF  ROCK  DEBRIS 

Maine  Experiment  Station  at  Orono,  the  fresh  and  highly 
polished  slaty  rock  was  found  but  a  few  feet  below  the  sur- 
face, proving  incontestably  that,  with  the  exception  of  the 
small  amount  of  organic  matter  that  had  since  been  added, 
not  an  ounce  of  the  soil  was  truly  native,  but  all  of  foreign 
birth,  and  a  mongrel  creature  of  compulsory  migration.  We 
shall  dwell  more  fully  upon  the  character  and  distribution  of 
these  soils  later.  The  single  illustration  above  given  will 
answer  present  purposes. 

In  a  less  degree  the  ice  along  the  shores  of  lakes  and  rivers 
may  exert  a  transporting  influence.  Thus  the  ice  first  formed 
along  the  shores  encloses  sundry  pebbles,  boulders,  and  sand. 
Through  the  expansion  force  of  the  freezing  water  as  the  entire 
surface  becomes  frozen  over,  this  shore  ice,  together  with  its 
enclosures,  may  be  pushed  up  some  distance  beyond  the  water 
line,  where  the  included  debris  is  deposited  on  melting.  Or, 
on  the  breaking  up  of  the  ice  in  the  spring,  the  shore  ice  may 
be  drifted  to  other  parts  of  the  lake,  or  down  the  stream,  per- 
haps for  miles  before  melting  sufficiently  to  cause  it  to  deposit 
its  load. 

(3)  Action  of  Wind.1  —  While  abrasion  by  the  wind  is  im- 
possible without  transportation,  the  converse  is  by  no  means 
true ;  indeed  it  is  as  an  agent  of  transportation  for  rock  detri- 
tus, without  appreciable  abrasion,  that  the  wind  accomplishes 
its  greatest  work,  though  in  like  manner  this  phase  is  most 
manifest  in  arid  regions. 

It  is  stated  by  Darwin  that  for  several  months  of  the  year 
large  quantities  of  dust  are  blown  from  the  northwestern  shores 
of  Africa  into  the  Atlantic  over  a  space  some  1600  miles  in 
width  and  for  a  distance  of  from  300  to  600  and  even  1000 
miles  from  the  coast.  During  a  stay  of  three  weeks  at  St.  Jago 
in  the  Cape  Verde  Archipelago,  this  authority  found  the  atmos- 
phere almost  always  hazy  from  the  extremely  fine  dust  coming 
from  Africa  and  falling  upon  the  land  and  water  for  miles 
around.  So  abundant  was  this  dust  that  a  distance  of  between 
300  and  400  miles  from  the  coast  the  water  was  distinctly 
colored  by  it.  In  the  arid  lands  of  Central  Asia  the  air  is  also 
reported  as  often  laden  with  fine  detritus  which  drifts  like  snow 
around  conspicuous  objects  and  tends  to  bury  them  in  a  dust 

1  See  article  on  Erosion  performed  by  the  Wind,  by  Professor  J.  A.  Udden, 
Journal  of  Geology,  Vol.  II,  1894,  p.  318. 


ACTION   OF   WIND  293 

drift.  Even  when  there  is  no  apparent  wind,  the  air  is  described 
as  often  thick  with  fine  dust,  and  a  yellow  sediment  covers 
everything.  In  Khotan  this  dust  sometimes  so  obscures  the 
sun  that  even  at  midday  one  cannot  see  to  read  fine  print  with- 
out the  aid  of  a  lamp.  The  tales  of  the  overwhelming  of  trav- 
ellers and  entire  caravans  by  sand  storms  in  the  Great  Desert 
of  Sahara  are  familiar  to  every  schoolboy.  Greatly  exagger- 
ated though  these  may  be,  the  accounts  of  Layard  and  of 
Loftus  show  us  that  the  sand  storms  which  are  of  frequent 
occurrence  during  the  early  part  of  summer  throughout  Meso- 
potamia, Babylonia,  and  Susiana  are  by  no  means  of  insignifi- 
cant proportions.  Layard  states  that  during  the  progress  of 
the  excavations  at  Nimrud,  whirlwinds  of  short  duration  but 
almost  inconceivable  violence  would  suddenly  arise  and  sweep 
across  the  face  of  the  country,  carrying  along  with  them  clouds 
of  dust  and  sand.  Almost  utter  darkness  prevailed  during 
their  passage,  and  nothing  could  resist  their  force  ;  the  Arabs 
would  cease  their  work  and  crouch  in  the  trenches  almost  suf- 
focated and  blinded  by  the  dense  cloud  of  fine  dust  and  sand 
which  nothing  could  exclude. 

The  accounts  of  Loftus  are  equally  impressive.  Describing 
their  departure  from  Warka  to  Sinkara,  he  says:  "A  furious 
squall  arose  from  the  southeast  and  completely  enveloped  us 
in  a  tornado  of  sand,  rendering  it  impossible  to  see  within  a 
few  paces.  Tellig  and  his  camels  were  as  invisible  as  though 
they  were  miles  distant.  A  continuous  stream  of  the  finest  sand 
drove  directly  in  our  faces,  filling  the  eyes,  ears,  nose,  and  mouth 
with  its  penetrating  particles,  drying  up  the  moisture  of  the 
tongue,  and  choking  the  action  of  the  lungs."  With  such 
descriptions  before  one  it  is  not  difficult  to  believe  that  these 
ruined  cities  have  in  the  course  of  centuries  been  completely 
hidden  and  their  sites  obscured  by  mounds  of  wind-drifted 
sand  and  dust. 

We  need  not,  however,  confine  ourselves  wholly  to  the  Old 
World  for  illustrations.  Not  longer  ago  than  May  of  1889  a 
dry  southwesterly  wind  which  for  several  days  had  prevailed 
in  various  parts  of  the  Northwest,  particularly  in  Dakota,  cul- 
minated in  a  storm  peculiarly  suggestive  from  a  geological 
standpoint.  It  is  stated1  that  during  the  prevalence  of  this 
wind,  on  the  6th  and  7th  of  the  month  mentioned,  the  air  be- 
1  American  Geologist,  June,  1889,  p.  398. 


294     TRANSPORTATION  AND  REDEFOSITION  OF  ROCK  DEBRIS 

came  filled  with  flying  particles  caught  up  from  the  ploughed 
fields,  fire-blackened  prairies,  public  roads,  and  sandy  plains. 
The  particles  formed  dense  clouds  and  rendered  it  as  impos- 
sible to  withstand  the  blast  as  it  is  to  resist  the  blizzard 
which  carries  snow  in  winter  over  the  same  region.  The  soil 
to  a  depth  of  4  or  5  inches  in  some  places  was  torn  up  and 
scattered  in  all  directions.  Drifts  of  sand  were  formed  in 
favorable  places,  several  feet  deep,  packed  precisely  as  snow- 
drifts are  packed  by  a  blizzard.  It  seemed  as  if  there  were 
great  sheets  of  dust  and  dirt  blown  recklessly  in  mid  air,  and 
when  the  wind  died  down  for  a  few  moments,  the  dirt,  fine 
and  white,  appeared  to  lie  in  layers  in  the  atmosphere,  clouding 
the  sun  and  hiding  it  entirely  from  sight  for  an  hour  or  more 
at  a  time.  (See  also  on  p.  184.) 

Over  the  wide,  dry,  and  bare  flat-topped  terraces  of  the  upper 
Madison  valley  the  wind  sweeps  in  a  strong  steady  current 
for  days  together,  or  during  the  heated  portion  of  the  year, 
when  the  sun  pours  from  a  cloudless  sky  its  hottest  rays  upon 
the  parched  soil,  starts  up  spasmodically  here  and  there  in  the 
form  of  small  whirlwinds  made  visible  by  the  dust  they  carry, 
and  which  wander  spectre-like  across  the  plain  to  noiselessly 
disappear  in  the  distant  mid  air. 

Dust  columns  of  this  nature  are  common  in  all  arid  regions, 
and  doubtless  have  been  observed  by  the  many  who  have 
crossed  the  Humboldt  desert  in  Nevada.  Seated  comfortably 
in  a  Pullman  car  on  the  Union  Pacific,  one  may  not  infrequently 
see  at  a  single  view  not  less  than  a  half  dozen  of  these  geologi- 
cal spectres,  each  in  the  distance  doing  its  apportioned  task 
and  silently  disappearing,  laying  down  its  load  of  sand  as  its 
strength  gives  out  and  leaving  it  for  its  successor.1 

Under  proper  conditions  such  of  these  wind-blown  sands  as 
are  too  heavy  to  be  carried  into  the  air  as  dust  accumulate 
upon  the  surface  in  the  form  of  drifts,  or  dunes,  all  lying  with 
their  longer  axes  approximately  at  right  angles  with  the  pre- 
vailing currents.  Excepting  during  periods  of  calm,  such  are 
in  a  state  of  almost  constant,  though  it  may  be  imperceptible, 
motion,  ever  changing  their  shapes  and  moving  onward  like 
long  parallel  drifts  of  snow.  The  rate  of  motion  of  a  dune 

1  Professor  J.  A.  Udden  estimates  that  the  dust  in  a  cubic  mile  of  lower  air 
during  a  dry  storm  weighs  not  less  than  225  tons,  while  in  severe  storms  it  may 
reach  126,000  tons  (Popular  Science  Monthly,  September,  1886) . 


ACTION   OF    WIND  295 

from  necessity  is  governed  by  the  strength  and  constancy  of 
the  winds,  and  the  fineness  and  dryness  of  the  sand.  Urged 
into  temporary  activity,  each  little  grain  goes  scurrying  up  the 
slope,  across  the  crest,  and  tumbles  to  rest  in  the  steeper 
declivity  upon  the  leeward  side,  to  be  slowly  buried  by  those 
which  follow.  This  is  the  sum  total  of  the  movement  taking 
place  in  the  march  of  a  dune,  whatever  its  pace  and  however 
great  its  bulk.  Yet  in  this  very  faculty  of  moving  itself  for- 
ward by  but  a  ten  billionth  part  of  its  bulk  at  a  time  lies  the 
whole  secret  of  its  power.  Silently,  imperceptibly  it  may  be 
except  when  measured  by  months  and  perhaps  years  of  time, 
retarded  by  no  walls  nor  ordinary  declivities,  it  relentlessly 
performs  its  task.1 

A  writer  in  one  of  the  recent  popular  magazines  estimates 
the  dunes  of  Hatteras  and  Henlopen  as  in  some  cases  upwards 
of  70  feet  in  height  and  moving  at  least  50  feet  a  year.  Swamps 
have  thus  been  filled,  forests  and  houses  buried,  and  it  is  stated 
that  but  a  few  years  can  elapse  before  the  entire  island  lying 
north  of  Cape  Hatteras  will  be  rendered  uninhabitable.  The 
sand  dunes  on  the  coast  of  Prussia  commenced  not  over  a 
century  ago,  and  already  fields  and  villages  have  been  buried 
and  valuable  forests  laid  waste  by  them.  In  one  instance  a 
tall  pine  forest  covering  many  hundred  acres  was  destroyed 
during  the  brief  period  intervening  between  1804  and  1827. 
Loftus,  writing  of  Niliyga,  an  old  Arab  town  a  few  miles  east 
of  the  ruins  of  Babylon,  says  that  in  1848  the  sand  began  to 
accumulate  about  it,  and  in  six  years  the  desert  within  a  radius 
of  six  miles  was  covered  with  little  undulating  domes,  while 
the  ruins  of  the  city  were  so  buried  that  it  is  now  impossible 
to  trace  their  original  form  and  extent.  A  still  more  striking 
illustration  of  the  rapidity  of  these  sand  accumulations  is  offered 
by  the  same  authority  in  describing  the  burial  customs  of  some 
of  these  ancient  people,  it  being  stated  that  the  earthen  coffins 
were  merely  stacked  in  layers  one  on  top  of  another,  and  left 
thus  to  be  covered  by  the  finer  sand  sifted  over  them  by  the 
winds  from  the  desert.  Even  Nineveh,  founded  some  twenty 
centuries  before  Christ  and  destroyed  1400  years  later,  became 
so  covered  by  drifted  sands  that  at  the  time  of  the  Greek 
Xenophon  (about  400  B.C.)  the  very  site  of  the  once  famous 

i  The  Wind  as  a  Factor  in  Geology,  Engineering  Magazine,  1892,  p.  596. 


296     TRANSPORTATION  AND  REDEPOSITION  OF  ROCK  DEBRIS 

city  was  unknown.  Marsh 1  gives  the  rate  of  movement  of  dunes 
along  the  western  coast  of  Jutland  and  Schleswig-Holstein  as 
averaging  13  J  feet  a  year,  while  Anderson  estimates  the  aver- 
age depth  of  the  sand  over  the  entire  area  as  about  30  feet, 
equalling  therefore  about  1|  cubic  miles  for  the  total  quantity. 

It  is  not  in  all  cases  possible  to  trace  the  drifted  sands  to 
their  various  sources.  Dunes  along  the  sea-coasts  are  in  nearly 
all  cases  composed  of  materials  thrown  up  by  the  waves  on 
the  beaches  in  the  immediate  vicinity.  This  is  the  case  with 
those  of  Hatteras,  Cape  Cod,  Gascony,  Algeria,  and  Schleswig- 
Holstein.  But  the  origin  of  the  large  inland  dunes,  like  those 
of  Nevada,  is  not  always  so  clear.  It  has  been  suggested  that 
these  last  are  formed  of  beach  sand  driven  in  by  the  prevail- 
ing westerly  winds  from  the  Pacific  coast.  This  is,  however, 
a  matter  of  very  grave  doubt,  and  it  seems  more  probable,  as 
stated  by  geologist  Russell,2  that  they  were  derived  from  the 
disintegrating  granites  of  the  Sierras.  They  certainly  have 
travelled  far,  and  are  not  a  product  of  disintegration  of  rocks 
in  the  immediate  vicinity.3 

By  wind  action,  accompanied  by  the  carrying  power  of  spas- 
modic or  perennial  streams,  were  formed  the  wide  stretches 
of  adobe  in  the  western  United  States,  and  according  to  many 
authorities  the  deposits  of  loess  which  cover,  as  in  Europe  and 
Asia,  areas  aggregating  many  square  miles  and  which  have  a 
depth,  in  extreme  cases,  of  2000  feet.4 

The  tendency  of  the  wind  is  not,  however,  in  all  cases  toward 

1  The  Earth  as  modified  by  Human  Action,  p.  562. 

2  Quaternary  History  of  Lake  Lahonton,  Nevada,  Monograph,  U.  S.  Geol. 
Survey,  1885. 

3  The  sands  covering  the  Egyptian  Sphinx  and  Pyramids  are  stated  to  have 
come  mainly  from  the  sea  on  the  north,  and  not  from  the  desert,  as  is  popularly 
supposed.     Sand  showers  having  their  origin  in  the  desert  of  Sahara  extend 
across  the  Mediterranean,  and  as  far  as  northern  Italy  (Nature,  July  18,  1889, 
p.  286). 

4  The  wind  plays  an  important  part  in  the  transportation  of  soils  in  Wyoming, 
owing  to  the  incoherent  state  of  the  soils,  due  to  the  lack  of  clay.    The  arid 
regions  of  this  state,  which  are  chiefly  Tertiary  and  Cretaceous  plains  and  table- 
lands, receive  very  little  rain.     Consequently  the  soils  become  loosened  by  great 
earth  cracks,  and  during  the  dry  and  windy  winter  weather  are  transported  in 
dense  clouds,  which  almost  suffocate  travellers,  to  the  broken  country  and  dis- 
tant hills  and  mountains.     In  a  single  season  it  is  not  an  uncommon  sight  to  see 
banks  of  earth,  like  huge  banks  of  snow,  behind  a  reef  of  rock,  or  in  the  lee  of 
large  bunches  of  sage  brushes  (U.  S.  Dept.  of  Agriculture,  Office  of  Experiment 
Stations,  Vol.  V,  No.  6,  1894,  p.  567). 


ACTION    OF    WIND  297 

forming  drifts  and  ridges,  but  at  times  rather  to  reduce  the  land 
to  one  general  level.  Thus  J.  Flinders  Petrie l  states  that  near 
the  ancient  cemetery  of  Tell  Nebesheh,  on  the  Isthmus  of  Suez, 
the  surface  of  the  country  has  been  cut  down  at  the  rate  of  4 
inches  a  century  until  some  8  feet  have  been  removed  from 
the  dry  areas  and  deposited  in  the  intervening  depressions, 
slowly  converting  the  existing  lakes  into  marshes,  and  the 
marshes  into  dry  land.  An  even  more  rapid  change  of  con- 
tours is  that  described  by  Dwight2  as  having  taken  place  on 
Cape  Cod,  Massachusetts.  The  entire  country  here  is  com- 
posed of  sand  so  susceptible  to  the  drifting  action  of  the  wind 
that  it  has  for  years  been  the  custom  of  the  people  to  sow  pines 
and  coarse  beach  grass  to  hold  it  in  place.  In  the  instance 
described  by  Dwight,  however,  reckless  pasturage  had  so  far 
destroyed  the  grass  as  to  lessen  its  protecting  power,  and 
under  the  strong  breezes  from  the  open  Atlantic  it  began  to 
drift  rapidly.  Over  an  area  of  about  1000  acres  the  sand  was 
blown  away  to  a  depth,  in  many  places,  of  10  feet.  "  Nothing," 
says  Dwight,  "  could  exceed  the  dreariness  of  this  scene.  Not  a 
living  creature  was  visible;  not  a  house,  nor  even  a  green  thing 
except  the  whortleberries  which  tufted  a  few  lonely  hillocks 
rising  to  the  height  of  the  original  surface,  and  prevented  by 
this  defence  from  being  blown  away  also.  The  impression  made 
by  this  landscape  cannot  be  realized  without  experience.  It 
was  a  compound  of  wildness,  gloom,  and  solitude.  I  felt 
myself  transported  to  the  borders  of  Nubia,  and  was  well 
prepared  to  meet  the  sand  columns  so  forcibly  described  by 
Bruce,  and  after  him  by  Darwin.  A  troup  of  Bedouins  would 
have  finished  the  picture,  banished  every  thought  of  my  own 
country,  and  set  us  down  in  an  African  waste." 

One  more  instance  of  contour  changes  of  this  sort  must  suffice.. 
It  is  stated  3  that  in  Pipestone  and  Rock  counties  in  Minnesota, 
the  bluffs  facing  to  the  westward  are,  as  a  rule,  more  precipi- 
tous and  more  rocky  than  those  facing  in  the  opposite  direc- 
tion. This  fact  is  regarded  by  Professor  Winchell  as  due  to 
the  action  of  the  prevailing  westerly  winds,  combined  with 
the  drying  effects  of  the  southwestern  sun  in  summer.  Such 
winds  would  uncover  and  keep  bare  the  coarser  materials  of 

1  Proc.  Royal  Geographic  Soc.,  November,  1889,  p.  648. 

2  Travels  in  New  England  and  New  York,  Vol.  Ill,  p.  101. 

3  Geol.  of  Minnesota,  Vol.  I,  p.  575. 


298     TRANSPORTATION  AND  REDEPOSITION  OF  ROCK  DEBRIS 

the  western  surface  by  blowing  away  the  sand  and  clay,  while 
the  bluffs  on  the  east  are  not  only  protected  from  the  winds, 
but  collect  upon  their  slopes  all  the  flying  particles  from  the 
prairies  above. 

The  finely  comminuted  rock  dust  blown  from  volcanic  vents 
is  often  drifted  for  long  distances  by  atmospheric  currents,  and 
ultimately  deposited  in  beds  of  no  insignificant  proportions. 
Dense  clouds  of  such  dust  were  blown  from  Icelandic  volcanoes 
to  the  coast  of  Norway  in  1875,  and  subsequent  to  the  eruption 
of  Krakatoa  (in  1883)  the  ship  Beacomfield  of  Philadelphia, 
while  at  a  distance  of  831  miles  from  the  source,  sailed  for  three 
days  through  clouds  of  dust  which  fell  upon  her  decks  at  the 
rate  of  an  inch  an  hour.  That  such  are  not  or  have  not  in 
the  past  been  unusual  instances  is  shown  by  results  obtained 
by  the  Challenger  Expedition,  volcanic  ashes  and  sand  being 
repeatedly  dredged  up  from  almost  abysmal  depths  at  points 
in  the  central  Pacific  far  remote  from  land  areas.  The  day 
following  the  explosive  eruption  of  St.  Vincent,  in  1812,  the 
Barbadoes  Island,  80  miles  to  the  windward,  was  completely 
shrouded  in  darkness  for  many  hours,  the  light  of  the  sun  being 
almost  wholly  obscured  by  the  cloud  of  impalpable  dust  which 
in  the  form  of  a  slow,  silent  rain  fell  over  the  whole  island. 
uThe  trade  wind  had  fallen  dead;  the  everlasting  roar  of  the 
surf  was  gone;  and  the  only  noise  was  the  crushing  of  the 
branches  snapped  by  the  weight  of  the  clammy  dust.  About 
one  o'clock  the  veil  began  to  lift,  a  lurid  sunlight  stared  in 
from  the  horizon,  but  all  was  black  overhead.  Gradually  the 
dust  cloud  drifted  away;  the  island  saw  the  sun  once  more, 
and  saw  itself  inches  deep  in  black,  and  in  this  case  fertiliz- 
ing, dust."  J 

1  Kingsley,  as  quoted  by  Belt,  in  The  Naturalist  in  Nicaragua,  p.  354. 


PART  V 

THE   RBGOLITH 

THROUGHOUT  the  millions  of  years  which  have  elapsed  since 
the  earth  assumed  its  present  form  and  essentially  solid  con- 
dition, the  rocks  composing  its  more  superficial  portions  have 
been  constantly  undergoing  degeneration  in  the  manner  de- 
scribed, and,  in  so  doing,  have  given  rise  to  the  immense  masses 
of  materials  which  constitute  the  thousands  of  feet  of  secon- 
dary rocks,  and  the  still  unconsolidated  sands,  gravels,  and  other 
products  which  will  be  considered  in  detail  later.  With  those 
products  which  have  undergone  lithification,  which  are  now  in 
the  state  of  consolidation  commonly  ascribed  to  rocks  by  the 
popular  mind,  we  shall  have  little  more  to  say.  These  have 
already  been  sufficiently  described  as  rocks  in  Part  II  of  this 
work.  It  is  to  the  most  superficial  and  unconsolidated  portion 
of  the  earth's  crust  that  we  will  now  devote  our  attention. 

Let  the  reader  for  a  moment  picture  to  himself  the  present 
condition  of  this  crust,  with  particular  reference  to  the  land 
areas.  Everywhere,  with  the  exception  of  the  comparatively 
limited  portions  laid  bare  by  ice  or  stream  erosion,  or  on  the 
steepest  mountain  slopes,  the  underlying  rocks  are  covered  by 
an  incoherent  mass  of  varying  thickness  composed  of  materials 
essentially  the  same  as  those  which  make  up  the  rocks  them- 
selves, but  in  greatly  varying  conditions  of  mechanical  aggrega- 
tion and  chemical  combination. 

In  places  this  covering  is  made  up  of  material  originating 
through  rock-weathering  or  plant  growth  in  situ.  In  other 
instances  it  is  of  fragmental  and  more  or  less  decomposed  mat- 
ter drifted  by  wind,  water,  or  ice  from  other  sources.  This 
entire  mantle  of  unconsolidated  material,  whatever  its  nature 
or  origin,  it  is  proposed  to  call  the  regolith,  from  the  Greek 
words  /07?709,  meaning  a  blanket,  and  Xt#o9,  a  stone.  Within 

299 


300 


THE   REGOLITH 


certain  limits  it  varies  widely  in  composition  and  structure,  and 
many  names  have,  on  one  ground  and  another,  been  applied  to 
its  local  phases,  the  more  important  of  which  are  given  in  tabu- 
lar form  below,  and  described  in  detail  in  the  pages  following. 
According  to  its  origin,  whether  the  product  of  transporting 
agencies  as  noted  above,  or  derived  from  the  degeneration  of 
rocks  in  situ,  the  regolith  is  found  lying  upon  a  rocky  floor  of 
little  changed  material,  or  becomes  less  and  less  decomposed 
from  the  surface  downward  until  it  passes  by  imperceptible 
gradations  into  solid  rock. 


The 
regolith 


Sedentary 


f  Residual  deposits 
[Cumulose  deposits 


Transported 


f  Residuary  gravels,  sands  and  clays, 
I     wacke,  laterite,  terra  rossa,  etc. 
Peat,  muck,  and  swamp  soils,  in 
part. 

f  Talus  and  cliff  debris,  material  of 
Colluvial  deposits  {     avalanches. 

Alluvial  deposits  (  Modern  alluvium,  marsh  and  swamp 
(including  aqueo-  -I  (paludal)  deposits,  the  Champlain 
glacial)  (  clays,  loess,  and  adobe,  in  part. 

f  Wind-blown  material,  sand  dunes, 
JEokan  deposits      {     adobe  and  loesg)  in  part> 

f  Morainal    material,    drumlins,    es- 
Glacial  deposits      {     kers,  osars,  etc. 


The  extreme  upper,  most  superficial  portion  of  this  regolith, 
that  which  affords  food  and  foothold  for  plant  life,  is  commonly 
designated  as  soil;  that  immediately  underlying  the  soil,  and 
passing  into  it  by  insensible  gradations,  is  known  as  the  sub-soil. 
This  last  differs  from  the  soil  proper  only  in  degree  of  compact- 
ness and  in  such  chemical  changes  as  may  have  been  induced 
in  the  soil  through  growing  organisms  and  more  extensive 
weathering.  Indeed,  the  soil  is  but  derived  from  the  sub-soil, 
and  were  it  entirely  removed,  would  shortly  be  replaced  through 
the  same  agencies  as  first  gave  it  birth. 

The  characteristics  of  individual  soils  can  be  best  discussed 
when  speaking  of  those  local  phases  of  the  regolith  of  which 
they  form  a  part,  and  with  this  understanding  we  will  proceed. 


1.     SEDENTARY  MATERIALS 

Here  are  to  be  considered  those  deposits  which,  resulting 
from  chemical  decomposition  or  disintegration,  from  any  or  all 
of  the  processes  involved  in  rock-weathering,  or  from  organic 


SEDENTARY  MATERIALS 


301 


accumulation,  are  found  to-day  occupying  their  original  sites. 
They  are,  in  fact,  the  primeval  types  of  nearly  all  soils  and  sec- 
ondary rocks,  since  those  of  drift  origin  are  but  derived  from 
sedentary  materials  through  the  transporting  agencies  of  air 
and  water.  They  may  be 
conveniently  divided  into 
two  classes,  (1)  residual1 
and  (2)  cumulose. 

(1)  Residuary  Deposits. 
Under  this  name,  then,  are 
included  all  those  prod- 
ucts of  rock  degeneration 
which  are  to-day  found  oc- 
cupying the  sites  of  the 
rock  masses  from  which 
they  were  derived,  and  im- 
mediately overlying  such 
portions  as  have  as  yet 
escaped  destruction.  The 

name  is  peculiarly  appro-  FIG.  26.  —  Showing  angular  outlines  of  residuary 
priate,  since  thev  are  actu-  Particles  from  decomposed  gneiss.  1,  mica ; 
*  .  ,  , J  c.  ,  ,  .  ,  2,  feldspar;  3,  quartz. 

ally   residues,  left  behind 

while  the  more  soluble  portions  have  been  leached   away  by 

meteoric  waters. 

The  residual  deposits  of  North  America  reach  their  maximum 
development  in  the  portion  of  the  United  States  east  of  the 
Mississippi  and  south  of  the  southern  margin  of  the  ice  sheet 
of  the  Glacial  epoch.  Their  mode  of  accumulation  and  gen- 
eral characteristics  have  been  very  thoroughly  discussed  by 
Professors  I.  C.  Russell,  Chamberlain,  and  Salisbury,2  on  whose 
papers  we  shall  draw  for  some  of  the  facts  given  here. 

1  Various  names  have  from  time  to  time  been  proposed  for  deposits  of  this 
nature,  but  obviously  it  is  impossible  to  include  under  a  single  lithological  term 
materials  so  widely  variable.    The  term  saprolite  (from  the  Greek  crairpos,  rotten, 
recently  suggested  by  G.  F.  Becker,  16th  Ann.  Rep.  II.  S.  Geol.  Survey,  Part  III, 
p.  289)  is  objectionable  as  conveying  the  idea  of  putridity.    The  old  provincial 
term  geest  adopted  by  De  Luc,  and  recently  endorsed  by  McGee  (llth  Ann. 
Rep.  U.  S.  Geol.  Survey,  1889-90,  p.  279),  has  lost  whatever  precise  meaning  it 
may  have  had,  being  defined  in  both  the  Standard  and  Century  dictionaries  as 
(1)  a  bed  derived  from  rock  decay  in  situ,  (2)  high  gravelly  land,  and  (3)  gravel 
or  drift.     The  term  gruss,  although  advocated  by  some  American  authorities,  is 
of  old  German  origin  and  open  to  the  same  objection. 

2  Bull.  52,  U.  S.  Geol.  Survey  and  Ann.  Rep.  U.  S.  Geol.  Survey,  1884-85. 


302  THE   REGOLITH 

The  prevailing  characteristic  of  an  old  residual  deposit,  from 
whatever  rock  it  may  be  derived,  is  a  ferruginous  clay.  Exam- 
ined by  a  microscope,  its  mineral  particles,  when  not  too  thor- 
oughly decomposed,  are  found  to  be  sharply  angular  in  outline. 
With  the  exception  of  the  quartz,  the  various  mineral  constitu- 
ents are  often  in  an  advanced  stage  of  decay,  and  the  more 
soluble  constituents  are  wholly  or  partially  lacking,  having  been 
leached  out,  in  the  manner  already  described. 

Owing  to  the  prevalence  of  the  aluminous  constituents,  these 
deposits,  when  thoroughly  decomposed,  as  on  the  immediate  sur- 
face, are  very  tenacious,  and  may  well  be  termed  clays.  Their 
colors  are  dull,  or  some  shade  of  brown  or  red,  owing  to  the 
higher  oxidation  and  perhaps  dehydration  of  the  ferruginous 
matter  set  free  by  the  decomposition  of  the  iron-bearing  sili- 
cate constituents.  Such  in  general  are  the  residual  soils  of  the 
southern  Appalachian  regions  of  the  United  States  and  which 
are  apparently  in  every  way  comparable  with  the  terra  rossa  of 
Europe,  but  only  in  a  slight  degree  with  the  laterite  of  India, 
to  which  they  have  often  unfortunately  been  referred.1  From 
a  chemical  standpoint  the  soils  forming  the  upper  portion  of  the 
residuary  deposits,  though  of  a  prevailing  aluminous  character, 
vary  widely  from  the  rock  masses  from  whence  they  were  de- 
rived, much  depending  upon  their  age  and  the  amount  of  actual 
decomposition  and  leaching  that  has  taken  place.  On  p.  306 
are  given  a  few  typical  but  widely  varying  analyses  which  will 
serve  to  illustrate  this  point. 

Deposits  of  this  nature  are  never  truly  stratified,  excepting 
where,  through  having  remained  wholly  undisturbed,  they  re- 
tain the  original  structure  of  the  parent  rock.  (See  under 
Effacement  of  Original  Characteristics,  p.  262.) 

The  residuary  differ  from  the  drift  deposits  in  that  they  con- 
tain no  materials  foreign  to  their  vicinity,  but  only  such  more 
enduring  matter  as  has  been  handed  down  to  them  from  the 

1  The  term  terra  rossa,  according  to  Neumayer  (Erdgeschichte,  Vol.  I,  p. 
405)  was  first  applied  to  the  red  residual  deposits  in  the  Karst  maritime  lands 
of  the  Adriatic  Sea.  The  material  is  described  as  a  highly  ferruginous  clay 
resulting  from  the  leaching  out,  by  meteoric  waters,  of  the  soluble  portions  of 
the  prevailing  limestones.  Its  distribution  is  by  no  means  limited  to  the  mari- 
time provinces  of  the  Karst,  but  it  is  found  also  on  the  Grecian  coasts  and  in 
the  Schwabia-Frankonia  Jura  Plateaus  of  Bavaria.  In  fact  it  is  to  be  found  any- 
where in  these  regions  where  the  prevailing  country  rock  is  a  marine  limestone 
and  erosion  not  sufficiently  active  to  remove  the  residuary  material. 


RESIDUARY   DEPOSITS  303 

parent  rock.  In  the  case  of  limestones  such  matter  consists 
mainly  of  aluminous  and  ferruginous  matter,  grains  of  sand, 
and  nodular  masses  of  chert  which  existed  as  mechanically 
admixed  impurities. 

The  inherited  characteristics  of  deposits  of  this  nature  may 
be  illustrated  by  the  accompanying  exaggerated  section  across 
central  Kentucky  where,  it  is  easy  to  see,  the  regolithic  mate- 
rial overlying  the  Lower  Silurian  and  Cambrian  limestones  may 


FIG.  27. 

contain  a  portion  of  all  the  insoluble  residues  from  the  hundreds 
of  feet  of  Upper  Silurian,  Devonian,  Lower  and  Upper  Carbonif- 
erous beds  which  formerly  stretched  above  them.  Upon  the 
nature  of  this  inheritance  must  depend  the  adaptability  of  the 
regolith  to  soil  purposes  and  its  consequent  fertility. 

The  transition  from  a  regolith  of  this  type  to  fresh  rock  is 
usually  quite  sharp,  owing  to  the  fact  that  limestones  decompose 
mainly  through  solution  from  the  immediate  surface.  Never- 
theless there  is  a  gradual  change  in  the  character  of  such  a 
deposit  from  above  downwards,  owing  to  the  oxidizing  influence 
of  the  air  and  percolating  waters.  (See  p.  307.) 

As  above  noted,  the  mineral  particles  in  the  older  residuary 
deposits  are,  with  the  exception  of  the  quartz,  found  to  be  as  a 
rule  in  a  state  of  advanced  decomposition.  Nevertheless  the 
ultimate  individual  constituents  of  even  the  darkest  clays  of  the 
driftless  regions  of  Wisconsin,  as  examined  by  Messrs.  Chamber- 
lain and  Salisbury,  are  transparent,  although  stained  by  iron 
oxides. 

Concerning  the  physical  properties  of  limestone  residues  as 
occurring  in  this  driftless  area,  the  following  statements  are 
made  by  Messrs.  Chamberlain  and  Salisbury.  "  Above,  the 
clay  graduates  into  soil  which,  outside  the  valleys,  is  uniformly 
shallow.  Beneath  the  soil,  the  clay  loses  the  dark  color  of  the 
latter,  due  to  the  presence  of  organic  matter,  but  is  for  a  certain 
distance  downward  not  unlike  the  superior  portion  in  texture. 
The  deeper  lying  clay,  where  limestone  is  the  subjacent  rock, 
is  the  most  characteristic  member  of  the  residuary  earth  series. 


304 


THE   REGOLITH 


It  is  not  like  that  above,  structureless,  although,  like  that,  it  is 
without  trace  of  stratification.  It  generally  shows  a  tendency  to 
cleave,  breaking  up  into  little  pieces  which  are  roughly  cubical. 
This  is  often  conspicuous,  and  especially  so  on  the  faces  of 

sections  which  are  thor- 
oughly dry.  In  such  sit- 
uations large  quantities 
of  the  clay  in  small  angu- 
lar blocks  may  be  removed 
by  slight  friction.  The 
size  of  the  cuboids  varies, 
within  somewhat  narrow 
limits,  from  a  small  frac- 
tion of  an  inch  to  one  or 
two  inches  in  diameter. 
This  cleavage  is  probably 
a  phenomenon  of  shrink- 
age due  to  drying,  as  it 
partially  disappears  when 

^ne  clay  becomes  wet. 
^^  structure  hag  giyen 

joint  '  clay,  an  appellation  not  alto- 


FIG. 28.  —  Showing  angular  character  of  quartz 
particles  in  decomposed  gneiss. 


rise  to  the  local  name  of 
gether  inappropriate. 

"  Upon  drying,  this  variety  becomes  very  hard  and  rock-like. 
It  only  becomes  adapted  to  serve  as  soil  by  surface  amelioration, 
as  is  shown  by  the  fact  that,  from  the  thousands  of  mineral  holes 
scattered  over  the  southern  part  of  the  mining  district,  the 
material  ejected  still  lies  beside  the  excavations  as  heaps  of  clay, 
without  covering  of  vegetation,  although  it  has  been  exposed  in 
most  cases  for  many  years.  Notwithstanding  this  fact,  the 
clay,  even  in  its  deepest  parts,  wherever  examined,  is  found  to 
abound  in  minute  perforations.  These,  in  many  cases  at  least, 
indicate  the  penetration  of  rootlets,  for  the  rootlets  themselves 
may  sometimes  be  found.  In  some  cases,  too,  the  perforations 
have  been  seen  to  undergo  a  gradual  variation  in  size,  and  to 
branch  now  and  then,  much  as  rootlets  do.  On  the  other  hand, 
it  is  probable  that  some  of  the  perforations  have  had  a  different 
origin,  for  in  one  case  a  small  insect  was  found  in  one  of  the 
little  canal-ways.  The  clay  is  exceedingly  tenacious,  and  hence 
the  perforations,  once  formed,  would  endure  for  long  periods  of 
time. 


RESIDUARY   DEPOSITS  305 

"  Another  characteristic  of  certain  portions  of  the  clay  is  its 
power  of  retaining  moisture.  It  can  rarely  be  found,  even  in 
the  driest  season,  unless  exposed  to  the  direct  rays  of  the  sun, 
without  visible  moisture  a  few  inches  from  the  surface.  The 
regions  where  it  is  present  are  conspicuously  less  affected  by 
drought  than  adjacent  localities  where  it  is  wanting.  For  this 
reason  it  is  a  valuable  sub-soil. 

u  Fragments  of  residuary  rock  are  not  uncommon  in  the  deeper 
portions  of  this  earth.  Of  these,  chert  fragments  are  most 
abundant,  and  occur  scattered  sparingly  throughout  the  clay  or 
sometimes  arranged  in  more  or  less  distinct  layers  in  it.  Even 
where  they  appear  to  be  entirely  wanting,  the  microscope  often 
reveals  minute  flakes  scattered  sparsely  throughout  the  clay. 
The  larger  pieces  are  more  numerous  near  the  basal  portion  of 
the  clay  than  higher  up. 

"  It  is  natural  to  suppose  that  the  residuary  earths  derived 
from  the  decomposition  of  limestone  would  differ  very  notably 
from  those  which  take  their  origin  from  sandstones  or  from 
shales  or  mixed  crystalline  rocks.  Yet  the  difference  is  far 
less  than  might  be  anticipated.  There  usually  overlies  the 
sandstone  strata  a  loamy  earth  not  very  far  removed  in  char- 
acter from  that  which  mantles  limestones.  It  is  somewhat 
more  sandy,  and  consequently  less  cohesive,  and  presents  the 
opposite  variations  in  vertical  sections,  becoming  less  cohesive 
below,  instead  of  more  so.  In  the  limestone  region  the  tough- 
est clay  lies  next  to  the  rock.  In  the  sandstone  regions  the 
soil  graduates  below  into  sand.  The  difference  is  most  con- 
spicuous where  the  mantle  has  been  washed  and  redeposited 
and  mingled  with  mechanically  derived  sand  and  secondary 
products,  as  occurs  in  some  of  the  valleys."1 

The  following  analyses,  in  part  from  this  same  report,  will 
answer,  in  connection  with  those  already  given,  to  show  the 
prevailing  type  of  the  residuary  deposits  throughout  widely 
separated  areas.  It  will  be  noted  that  silica  exceeds  as  a  rule 
all  other  constituents,  while  alumina,  iron  oxides,  and  moisture 
make  up  the  main  bulk  of  the  residue.  This  generalization 
holds  good  of  nearly  all  sedentary  soils,  whatever  the  character 
of  the  rocks  from  which  they  were  derived,  and  is  the  more 
pronounced  the  more  advanced  the  decomposition. 

1  6th  Ann.  Rep.  U.  S.  Geol.  Survey,  1884-85,  pp.  240-242. 
x 


306 


THE   REGOLITH 


I-H   t-   OS   CO   CO 

O   CM   l>-   O   CO 
O   I-H   O  -^   t-^ 


t-     .  O 


O   O   O   O   CO 


CO    00 


1  a 

r*-1    CO 


t*—  lO        .   CO     •   CO   Oi   CO   QO   ^H   i-H 

dco"   CM    "o"   'coooo't^cM 

^   .-H      rH  CM 

CM  !>•    O      •    •    •  O   iO   tr-  CM   CO 

I-H   -^   rH   CO   OO    •    •      00 

OCM     CO      I    I    I  O  rH  O   C4  OS 
iO   C^     A 

.s 

COCO'^COCO'^tlCOCMCMCOOCMOSOS  CX) 

I-H      O      O      OS      rH      ^      O      CM      OS      CO      CO      t^-      CO      O  CO 

>-<OOOOl-Hl-Hl-Hi-Hl-HOrH  O 

COCOCOCOCOCO»OCO»OCOOSOSCM  OS 

i-^OOOST^I^cOIr^cMCM  O 

COi-HCOOOOOOrHrHO'o'od 
vo      CM  I-H 

'5* 

g^OSt^COCOi-HCOCOOCOCOOTfl 
>COi-H(MCMOOOSI>-OOOS'H^COCO 

oscdt^oddo'ddoo'dod 

^ 

^ss^^sss^ssg^s 

rHCMOoddoddcM'i-H-Hido 

e 

•      •      •      •   Q  ^ 

^IIIsf  :;:::?: 

•O^^aj^.2  O  Cx 

31      V-/       ®       !J    !         3       S  ^  '        s-^  '       rrt 

/T^  <s    CB   "^    'S    ^    ^^-^  x— N  S    /~x  o   i^  -2 

0-2   g   So   8  P  *  5  S  2-  I  ^ 

a  I  §  §  1 1  1  a  1 1  r  &  -I  r 

ill ii tig  Infill 

g^SSHgsas.gSfeSl 


8 


ll 

S  t3 


33 

G  fl 
O  O 

O  O 


RESIDUARY   DEPOSITS  307 

Columns  I,  II,  III,  and  IV  of  this  table  (see  opposite  page) 
are  limestone  residuals  from  southern  Wisconsin.  Columns  I 
and  II  are  from  the  same  vertical  section,  I  being  4J  feet  from 
the  surface,  and  II  8J,  and  in  contact  with  the  underlying  lime- 
stone. Columns  III  and  IV  are  similarly  related,  III  being  3 
feet  from  the  surface,  and  IV  4J  feet,  the  lower  sample  lying  on 
the  unchanged  rock.  The  larger  percentages  of  silica  in  the 
samples  from  nearest  the  surface  indicate  a  higher  state  of 
decomposition,  the  soluble  portions  having  been  more  largely 
removed.  The  presence  of  larger  percentages  of  alkalies  in 
these  same  samples  indicates  that  these  salts  existed  in  the  form 
of  silicates  which  have  resisted  the  decomposing  influences,  and 
remain  mechanically  included  in  the  residues.  Column  V  is  a 
clay  from  the  decomposition  of  the  Knox  dolomite  at  Morris- 
ville,  Alabama ;  VI  the  characteristic  red  earth  from  the  decom- 
position of  coralline  limestone  on  the  islands  of  Bermuda;  VII 
a  product  of  the  decay  of  a  diabase  dike  at  Wadesboro,  North 
Carolina;  VIII  a  gabbro  sub-soil  from  Maryland;  IX  a  sub-soil 
from  the  decomposition  of  Trenton  limestone  near  Hagerstown, 
Maryland ;  and  X  a  residual  soil  from  the  decomposition  of  a 
Triassic  sandstone,  Maryland. 

A  microscopic  examination  of  the  material  represented  by 
analyses  I  and  IV,  as  given  by  the  authorities  quoted,  showed 
it  to  consist  of  particles  in  an  extreme  condition  of  comminu- 
tion. An  actual  measurement  of  over  700,000  of  these  particles 
yielded  results  as  below  :  — 

Particles  less  than  .0025  mm.  in  diameter 721.866  % 

Particles  between  .0025  mm.  and  .005  mm.  in  diameter 9.812 

Particles  over  .005  mm.  in  diameter    .  0.634 


732.312  % 

Of  those  over  .005  millimetre  in  diameter,  particles  reaching 
0.06  millimetre  were  not  rare.  Nearly  all  those  above  0.1  milli- 
metre were  found  to  be  of  flints  and  cherts  which  graded  up 
into  chips  and  flakes  of  notable  sizes.  Particles  much  coarser 
than  those  above  enumerated  do  indeed  occur,  but  their  actual 
number  is  comparatively  small,  though  their  comparative  bulk 
may  be  considerable. 

Work  of  a  like  nature,  but  done  under  somewhat  different 
conditions,  by  Dr.  Milton  Whitney,  showed  the  residues  from 
the  Trenton  limestones  near  Hagerstown,  Maryland,  to  contain 


308 


THE   REGOLITH 


on  an  average  some  45%  of  finely  comminuted  material,  the 
individual  particles  of  which  vary  in  size  between  .005  and 
.0001  millimetre  in  diameter,  and  which  may  appropriately 
be  termed  clay.  As  Dr.  Whitney  has  calculated,  there  are 
approximately  22,000,000,000  grains  of  sand  and  clay  in  each 
gramme  of  such  a  sub-soil,  presenting  in  every  cubic  foot  not 
less  than  158,000  square  feet  of  surface  to  the  action  of  water 
and  air,  as  well  as  to  the  roots  of  growing  plants. 

The  results  of  mechanical  analyses  of  (I  and  II)  resi- 
dues from  the  Trenton  limestone,  (III)  Triassic  sandstone, 
(IV)  gabbro,  and  (V)  gneiss  are  presented  in  tabular  form 
below.1 


DIAMETER 

OF 

PARTICLES 

MM. 

CONVENTIONAL  NAMES 

I 

II 

III 

IV 

V 

2-1 

0.54 

0.17 

0.00 

0  00 

0.19 

1-.5 

Coarse  sand     

0.32 

0.00 

0.23 

0.26 

1.80 

.5-.25 

Medium  sand  . 

0  72 

0  15 

1  29 

0  18 

3  12 

.25-.! 

Fine  sand    .... 

0  62 

0  25 

4  03 

0  66 

6  96 

.1-.05 

Very  fine  sand     .... 

4.03 

2.34 

11  57 

6  73 

8  76 

.05-  01 

Silt 

36  02 

19  04 

38  97 

47  32 

34  92 

.01-.005 

Fine  silt      

14.99 

20  88 

8  84 

10  04 

12  14 

.005-.0001 

Clay   

41  24 

51  77 

32  70 

34  90 

28  82 

Total  mineral  matter  .... 
Organic  matter,  water,  and  loss 

88.48 
1.52 

94.60 
5.40 

97.63 
2.37 

94.44 
5.56 

96.71 
3.29 

100.00 

100.00 

100.00 

100.00 

100.00 

Many  of  the  products  of  weathering  of  siliceous  crystalline 
and  calcareous  rocks  are  of  economic  importance  as  soils,  clays, 
and  iron  ores,  as  elsewhere  noted.  The  kaolin  beds  of  northern 
Delaware  and  southwestern  Pennsylvania  are  mainly  decom- 
posed, highly  feldspathic,  gneissic  rocks,  and  which  as  dug 
from  the  pits  still  retain  their  gneissic  structure,  but  which 
are  now  plastic  clays  full  of  angular  quartz  fragments,  mica 
scales  and  feldspar  particles  in  various  stages  of  decomposi- 
tion. The  change  that  has  taken  place  consists  in  a  kaoliniza- 
tion  of  the  feldspars,  whereby  the  alkalies  are  largely  removed, 
and  a  residue  consisting  essentially  of  a  hydrous  silicate  of 

1  Bull.  No.  21,  Maryland  Agricultural  Exp.  Station,  by  Milton  Whitney,  1893. 


RESIDUARY   DEPOSITS 


309 


alumina  left  in  their  place.  The  quartz  granules  are  disaggre- 
gated, and  their  surfaces  sometimes  slightly  etched  by  the 
action  of  the  alkaline  car- 
bonates ;  the  black  mica, 
where  such  existed,  de- 
composed, giving  rise  to 
rust-colored  spots.  The 
material  is  dug  from  the 
pits  and  washed  with  wa- 
ter to  separate  the  im- 
purities, the  "  kaolin  "  or 
clay  remaining  in  sus- 
pension, and  being  ulti- 
mately saved  by  nitration 
through  canvas.  This 
finest  material,  as  seen 
under  the  microscope,  still 
contains  particles  of  un-  FIG.  29.  —  Showing,  on 
decomposed  feldspars  and 
shreds  of  white  mica,  to- 
gether with  other  extremely  irregularly  outlined,  sometimes 
almost  amoeba-shaped  forms,  as  shown  in  Fig.  29.  An  average 
of  two  mechanical  analyses  of  this  clay,  made  under  Dr.  Whit- 
ney's direction,  yielded  the  results  given  below  :  — 


the  left,  the  mineral 
kaolinite  as  seen  under  the  microscope,  and 
on  the  right,  washed  kaolin. 


MOISTURE  IN  AIR- 
DRY  MATERIAL  AT 
100°  C. 

MOISTURE  ON 
IGNITION 

SILT 

.05-.01    MM. 

FINE  SILT 
.01-.005  MM. 

CLAY 
.005-.0001  MM. 

0.41% 

11.41  % 

31.79% 

7.31  % 

47.78% 

Chemical  analyses  of  the  same  material,  made  in  the  labora- 
tories of  the  United  States  Geological  Survey,  yielded :  - 

Silica  (Si02) 48.73% 

Titanic  oxide  (Ti02) 0.17 

Alumina  (A1203) 37.02 

Ferric  iron  (Fe203) 0.79 

Lime  (CaO) 0.16 

Magnesia  (MgO) 0.11 

Potash  (K20) 0.41 

Soda  (Na20) 0.04 

Water  at  100° 0.52 

Ignition 12.83 

Phosphoric  acid  (P205) 0.03 

100.81  % 


310  THE   REGOLITH 

Among  the  special  names  that  have  from  time  to  time  been 
given  to  local  phases  of  residuary  accumulations,  there  remain 
two,  the  laterite  and  wacke,  which  are  sufficiently  common  to 
merit  some  attention.  The  first  mentioned  of  these,  laterite, 
like  loess  and  several  other  terms  that  might  be  mentioned, 
has  to  a  considerable  extent  lost  its  true  lithological  signifi- 
cance through  careless  usage.  Originally  the  name  was  applied 
to  a  vesicular  highly  ferruginous  clay,  soft  in  the  mass,  but  hard- 
ening on  exposure  to  the  weather,  and  which  has  a  wide  distribu- 
tion throughout  India  and  Ceylon.  Two  forms  are  commonly 
recognized,  —  the  one  capping  the  summits  of  hills  and  plateaux 
on  the  highlands  of  central  and  western  India,  and  underlain 
by  the  Deccan  traps;  and  the  second  occurring  on  the  lowlands, 
in  part  overlying  gneisses  and  granites.  The  prevailing  colors 
of  the  laterite,  when  freshly  broken,  are  various  tints  of  brown, 
red  and  yellow  mottled,  or  whitish  ;  after  exposure  it  is  usually 
covered  with  a  brown  or  blackish  brown  coating  of  limonite. 
When  first  dug  out,  the  material  is  sufficiently  soft  to  be  cut 
with  a  pick  or  shovel,  but  becomes  greatly  indurated  on  expos- 
ure. In  some  instances  the  material  is  of  so  compact  a  texture 
and  so  hard  as  to  resemble  jasper.  In  many  forms  of  laterite 
the  material  is  traversed  by  "small  irregular  tortuous  tubes 
from  a  quarter  of  an  inch  to  upwards  of  an  inch  in  diameter." 
These  penetrate  the  mass  in  all  directions,  though  most  com- 
monly nearly  vertical,  and  are  often  lined  with  a  coating  of 
limonite.  On  weathering,  these  give  rise  to  extremely  irregu- 
larly pitted  or  scoriaceous  surfaces,  which,  together  with  the 
dense,  often  botryoidal  structure,  cause  it  to  resemble  certain 
types  of  igneous  rocks,  for  which  it  has  more  than  once  been 
mistaken.  The  more  massive  forms  show  usually  a  horizontal 
banding.  Some  forms  of  laterite  show  a  brecciated  structure, 
due  to  its  detrital  fragments  becoming  recemented  into  masses 
closely  resembling  the  original  rock.  The  high  level  form,  that 
which  occurs  capping  the  hills  and  plateaux  on  the  highlands 
of  central  and  western  India,  is  fine  grained  and  compact  and 
of  a  fairly  homogeneous  structure,  although  the  iron  oxide  may 
be  somewhat  irregularly  distributed  and  sometimes  segregated 
in  pisolitic  nodules  sufficiently  abundant  to  form  an  ore.  The 
lower  level  form,  that  which  covers  large  areas  of  both  east  and 
west  coasts,  frequently  contains  grains  of  sand  and  pebbles 
embedded  in  a  ferruginous  matrix.  It  is,  as  a  rule,  less  homo- 


LATERITE   AND    WACKE 


311 


geneous  than  the  high  level  form,  but  nevertheless  passes  into  it 
by  insensible  gradations. 

The  origin  of  both  high  and  low  level  forms  of  the  laterite 
has  been  the  subject  of  much  speculation.  It  is  probable  that 
all  of  it  is  of  a  residual  nature,  i.e.  represents  the  less  soluble 
portions  of  pre-existing  rock  masses.  That  which  is  found  on 
the  high  levels  occurs  overlying  the  Deccan  trap  sheets,  into 
which  it  can  in  many  instances  be  traced,  proving  conclusively 
its  origin  from  this  rock  by  the  ordinary  processes  of  weather- 
ing. The  low-lying  variety  can,  in  many  instances,  in  like  man- 
ner be  traced  back  to  its  origin  from  more  siliceous,  gneissic,  and 
granitic  rocks.  A  part  of  the  material,  however,  has  the  ap- 
pearance and  structure  of  a  clastic  rock  of  sedimentary  origin, 
and  so  it  is  considered  by  the  best  authorities  to  be. 

The  chemical  composition  of  a  very  ferruginous  laterite  from 
Rangoon  is  as  below  :  — 


CONSTITUENTS 

INSOLUBLE 

SOLUBLE 

BULK 

Silica  (SiC>2) 

30.728% 
[•    2.728     j 

I    6.802 

6.848% 
5.783 
46.279 
0.742 

0.090 

37.576% 
I  52.802 

6.892 

Alumina  (AloOs) 

Iron  sesquioxide  (I^Os) 

Lime  (CaO)     

Magnesia  (MgO)  

Alkalies  

\Vater  and  loss 

40.258 

59.742 

100.00  % 

100.00% 

"  The  surface  of  the  country  composed  of  the  more  solid 
forms  of  laterite  is  usually  very  barren,  the  trees  and  shrubs 
growing  upon  it  being  thinly  scattered  and  of  small  size.  This 
infertility  is  due,  in  great  part,  to  the  rock  being  so  porous  that 
all  the  water  sinks  into  it,  and  sufficient  moisture  is  not  retained 
to  support  vegetation.  The  result  is  that  laterite  plateaux  are 
usually  bare  of  soil,  and  frequently  almost  bare  of  vegetation."1 

Wacke  is  an  old  German  name  now  but  little  used,  designat- 
ing the  gray,  brown  to  black  earthy  residue  or  clay  resulting 
from  the  decomposition  in  place  of  basic  eruptive  rocks,  as 


1  Manual  of  the  Geology  of  India,  by  R.  D.  Oldham,  2d  ed.,  1893,  pp.  369-390. 


312  THE   REGOLITH 

basalt,  melaphyr,  etc.  In  composition  the  material  naturally 
varies  with  the  character  of  the  rock  from  which  it  was  derived, 
and  the  amount  of  decomposition  and  leaching  it  may  have 
undergone. 

It  seems  advisable  to  call  attention  here,  a  little  more  emphati- 
cally, to  the  fact  that  the  same  processes  which  in  ages  past 
have  been  instrumental  in  the  formation  of  sandstones,  shales, 
slates,  or  marls  are  to-day,  and  have  in  late  Tertiary  and  in 
Quaternary  times,  given  us  soils;  in  other  words,  many  of  our 
soils  are  but  secondary  rocks  in  a  state  of  loose  consolidation, 
and  many  of  the  accumulations  classed  as  residual  were  de- 
rived by  disintegration,  in  situ,  of  alluvial  materials  ;  materials 
brought  down  years  ago  and  deposited  in  shallow  seas.  The 
amount  of  consolidation  undergone  by  the  more  recent  of  these 
sediments  has  in  many  instances  been  so  slight  that  on  elevation 
above  the  water  level  they  are  ready  almost  at  once  to  assume 
the  role  of  soil  with  little  if  any  preparatory  disintegration. 
Nevertheless  consistency  demands  that  .such  be  here  grouped 
as  residuary. 

Over  what  is  known  as  the  coastal  plain  of  the  middle  Atlan- 
tic slope,  a  narrow  belt  bordering  on  the  Atlantic  and  extending 
from  the  Hudson  River  on  the  north  to  the  Roanoke  on  the 
south,  have  been  deposited  in  late  Mesozoic  and  Tertiary  times 
a  series  of  gravels,  sands,  and  clays  which  constitute  the  well- 
known  Potomac,  Appomattox,  and  Columbian  formations  of 
Darton,  McGee,  and  others.  These  are  all  detrital  deposits 
from  the  eastern  Appalachian  regions,  brought  down  by  streams 
and  deposited  in  the  shallow  estuaries  and  deltas  of  these 
periods,  but  which  have  remained  in  a  condition  of  slight  con- 
solidation, and  through  subsequent  elevation  and  weathering 
form  the  soils.  Such  vary  widely  and  abruptly.  In  the  region 
northeast  of  Washington,  the  Potomac  formation  consists  of 
feldspathic  sands,  gravels,  and  clays  irregularly  bedded  and 
often  enclosing  notable  accumulations  of  rounded  pebbles  of 
quartzite  brought  down  from  the  Appalachian  and  Piedmont 
regions.  The  Appomattox  formation,  from  which  was  derived 
surface  soil  in  the  vicinity  of  the  Rappahannock  and  Appomat- 
tox in  Virginia,  is  a  yellowish  or  orange-colored  clay  and  sand 
with  sometimes  interbedded  gravel.  The  Columbian  formation, 
which  yields  the  surface  soil  of  the  main  portion  of  Washington 
City  and  the  immediate  valley  of  the  Potomac  and  contributary 


CUMULOSE   DEPOSITS 


313 


streams  southward,  is  a  delta  and  littoral  deposit  made  up  of 
materials  worked  over  from  the  older  Potomac  and  Lafayette 
formations  and  also  of  granitic  sands  and  clays  from  the  decom- 
posed rocks  of  the  Piedmont  plateau. 

The  clays  of  the  Potomac  formation  above  mentioned  are  not 
infrequently  sufficiently  homogeneous  and  plastic  to  be  utilized 
in  the  manufacture  of  brick,  tiles,  and  pottery.  The  following 
table  shows  the  finely  comminuted  condition  of  the  materials 
which  go  to  make  up  these  clays  in  Maryland,  as  determined 
by  Whitney.1 


DIAMETER 

MM. 

CONVENTIONAL  NAMES 

EED  CLAY, 
TILE 

EED  CLAY, 
PUDDLING 

BLUE  CLAY, 
STONEWARE 

2-1 

Fine  gravel                           ... 

0.00  % 

0.31  % 

0.00  % 

1-5 

Coarse  sand                

0.00 

0.82 

0.00 

.5-.25 

Medium  sajnd    

0.50 

2.69 

0.29 

.25-.! 

Fine  sand     

2.63 

3.23 

1.27 

1-05 

Very  fine  sand                          . 

962 

889 

8.93 

.05-.01 

Silt  

25.13 

26.17 

20.16 

01-  005 

Fine  silt 

13.44 

11.18 

16.72 

.005-.0001 

Clay     

42.34 

42.36 

50.02 

Total    

93.76  % 

95.65  % 

97.39  % 

Organic  matter,  water  loss     .     . 

6.24 

4.35 

2.61 

(2)  Cumulose  Deposits.  —  To  be  classed  with  the  sedentary 
deposits,  in  that  they  result  from  the  gradual  accumulation  of 
material  in  situ,  but  differing  radically  in  both  composition  and 
origin  from  those  just  described,  are  those  portions  of  the  rego- 
lith  which  result  from  the  gradual  accumulation  of  organic 
matter  with  only  small  amounts  of  foreign  detritus ;  which  are 
made  up  almost  wholly  of  the  combined  accumulations,  organic 
and  inorganic,  of  growing  plants.  Such  may  not  infrequently 
be  found  in  all  stages  of  formation,  in  enclosed  ponds  or  lakes, 
without  appreciable  inlet  or  outlet,  being  merely  due  to  stand- 
ing water  in  low  places.  "  Such  pools,  when  not  exposed  to 
periodical  drying  up,  are  invaded  by  a  peculiar  vegetation,  first 
mostly  composed  of  confervse,  simple  thread-like  plants  of  vari- 
ous color  and  of  prodigious  activity  of  growth,  mixed  with  a 
mass  of  infusoria,  animalcules,  and  microscopic  plants,  which, 


.  4,  U.  S.  Dept.  of  Agriculture,  1892. 


314  THE   KEGOLITH 

partly  decomposed,  partly  containing  the  floating  vegetation, 
soon  fill  the  basins  and  cover  the  bottom  with  a  coating  of 
clay-like  mould.  So  rapid  is  the  work  of  these  minute  beings, 
that  in  some  cases  from  6  to  10  inches  of  this  mud  is  deposited 
in  one  year.  Some  artificial  basins  in  the  large  ornamental 
parks  of  Europe  have  to  be  cleaned  of  such  muddy  deposits 
of  floating  plants,  mixed  with  small  shells,  every  three  or  four 
years. 

"  When  left  undisturbed,  this  mud  becomes  gradually  thick 
and  solid  ;  in  some  cases,  of  great  thickness  ;  affording  a  kind  of 
soil  for  marsh  plants,  which  root  at  the  bottom  of  the  basins  or 
swamps  and  send  off  their  stems  and  leaves  to  the  surface  of 
the  water  or  above  it ;  where  their  substance  becomes  in  the 
sunshine  hard  and  woody. 

"  As  these  plants  periodically  decay,  their  remains  of  course 
drop  to  the  bottom  of  the  water  ;  and  each  year  the  process  is 
repeated,  with  a  more  or  less  marked  variation  in  the  species 
of  the  plants.  After  a  time  the  basins  become  filled  by  these 
successive  accumulations  of  years  or  even  centuries,  and  the 
top  surface  of  the  decayed  matter,  being  exposed  to  atmospheric 
action,  is  transformed  into  humus  and  is  gradually  covered  by 
other  kinds  of  plants,  making  meadows  and  forests.  In  other 
cases  when  basins  of  stagnant  water  are  too  deep  for  vegetation 


FIG.  30.  —  Section  across  a  small  lake,    a,  bed  rock  ;  66,  drift ;  cc,  growing  peat ; 
dd,  decaying  peat ;  ee,  climbing  bog. 

of  aquatic  plants,  nature  attains  the  same  result  by  a  different 
special  process  ;  namely,  by  the  prolonged  vegetation  of  certain 
kinds  of  floating  mosses,  especially  the  species  known  as  sphagna. 
These  grow  with  prodigious  speed,  and  expanding  their  branches 
•  in  every  direction  over  the  surface  of  ponds  or  small  lakes,  soon 
cover  it  entirely.  They  thus  form  a  thin  floating  carpet,  which 
as  it  gradually  increases  in  thickness  serves  as  a  solid  soil  for 
another  kind  of  vegetation,  —  that  of  the  rushes,  the  sedges,  and 
some  kinds  of  grasses,  which  grow  abundantly  mixed  with  the 
mosses,  and  which  by  their  water-absorbing  structure  furnish 


CUMULOSE   DEPOSITS 


315 


a  persistent  humidity  sufficient  for  the  preservation  of  their 
remains  against  aerial  decay.  The  floating  carpet  of  moss  be- 
comes still  more  solid,  and  is  then  overspread  by  many  species 
of  larger  swamp  plants,  and  small  arborescent  shrubs,  especially 
those  of  the  heath  family ;  and  so,  in  the  lapse  of  years,  by  the 
continual  vegetation  of  the  mosses,  which  is  never  interrupted, 
and  by  the  yearly  deposits  of  plant  remains,  the  carpet  at  last 
becomes  strong  enough  to  support  trees,  and  is  changed  into  a 
floating  forest,  until,  becoming  too  heavy,  it  either  breaks  and 
sinks  suddenly  to  the  bottom  of  the  basin,  or  is  slowly  and  grad- 
ually lowered  into  it  and  covered  with  water."  1 

It  is  to  such  processes  that  are  due,  in  large  part,  the  inland 
swamp  soils  of  many  localities.  Beginning  at  and  near  the 
shore  and  upon  a  soil  of  wet  sand,  the  organic  matter  has  accu- 
mulated year  by  year  till  now  several  feet  in  thickness  and  in 
some  cases  covering  miles  of  territory.  The  proportion  of  or- 
ganic matter  in  such  a  deposit  naturally  increases  from  the  shore 
outward  until  in  the  upper  and  central  layers  it  may  comprise 
90  %  of  the  total  weight. 

This  feature  is  well  brought  out  in  the  following  analyses  of 
material  from  an  open  ground  prairie  swamp  in  Carteret  County, 
North  Carolina. 


CONSTITUENTS 

I 

II 

Silica  (insoluble)  (Si02) 

80.84% 

1.62% 

Silica  (soluble)  (SiO  ) 

3.70 

0.00 

Alumina  (A^Os)                    .          .          ... 

2.69 

0.39 

Oxide  of  iron  (Fe20s)      . 

1.18 

0.15 

Lime  (CaO)             

0.44 

0.36 

Magnesia  (MgO)     

0.22 

0.14 

Potash  (K20)      

0.07 

0.06 

Soda  CNa.O)  . 

0.02 

0.13 

Phosphoric  acid  (P^Og) 

0.08 

0.06 

Sulphuric  acid  (SOg)                 .              ... 

0.06 

0.00 

Chlorine  (Cl)                         

Trace 

0.02 

Organic  matter  (C)     .              

7.70 

87.25 

Water  (H2O) 

2.50 

9.60 

Column  I  of  the  above  is  from  the  margin  —  the  oak  fringe  — 
of  this  great  swamp,  near  North  River,  about  8  miles  north  of 

1  Geol.  Survey  of  Pennsylvania,  1885,  p.  106. 


316  THE   REGOLITH 

Beaufort ;  it  is  light  gray  to  ash-colored  with  a  growth  of  white 
oak,  gum,  maple,  pine,  and  palmetto  trees ;  the  situation  is  low 
and  flat.  "  This  margin  belt  of  semi-swamp  is  from  a  half  mile 
or  less  in  width  to  above  a  mile.  The  surface  rises  towards  the 
interior  and  is  covered  by  a  soil,  if  it  may  be  called  such,  repre- 
sented by  column  II,  which  is  2  to  3  feet  deep  and  upwards,  and 
lies  on  a  bed  of  white  sea-sand.  It  consists  of  a  loose  open  mass 
of  half-decayed  woody  matter,  of  a  brown  color,  and  is  in  fact 
a  superficial,  uncompressed  lignite  ;  for  it  will  be  observed  that 
the  analysis  includes  nearly  10  %  of  water,  so  that  the  dry  sub- 
stance would  give  but  3|-  %  of  inorganic  matter,  not  more  than 
would  be  accounted  for  by  the  ash  of  the  woody  matter.  The 
growth  is  a  dense  thicket  of  spindling  shrubs  with  small  scat- 
tered maples  and  bays."1 

Wiley  has  described  2  deposits  of  a  somewhat  similar  nature 
as  covering  1,000,000  acres  in  the  Kissimmee  valley  of  Florida. 
These,  which  are  of  a  dark  brown  to  deep  black  color,  contain 
in  some  cases  as  much  as  96.16%  of  volatile  matter,  aand  vary 
from  3  to  20  feet  in  depth.  Such,  when  properly  drained,  may 
be  made  extremely  fertile,  though  in  periods  of  drought  endan- 
gered by  fire  which,  once  started,  may  burn  for  months,  doing 
immense  damage.  The  partially  reclaimed  areas  of  the  Great 
Dismal  Swamp  of  Virginia  are  fair  representative  types  of 
swamp  soils. 

The  formation  of  cumulose  deposits  is  not,  however,  limited 
to  lakes,  stagnant  ponds,  or  even  to  swamps  as  the  word  is  ordi- 
narily used,  excepting  as  the  swamp  itself  may  be  incidental 
and  consequent.  Regions  of  poor  drainage,  particularly  in 
moist  and  cool  climates,  may  give  rise  to  growths  of  sphagnous 
mosses  and  subsequently  to  plants  of  a  higher  type,  which  in 
course  of  years  assume  no  insignificant  proportions. 

In  accounting  for  such  accumulations,  we  have  but  to  remem- 
ber that  ordinarily  when  a  plant  dies,  its  organic  constituents 
are  returned  to  the  atmosphere  once  more  in  a  comparatively 
brief  period  of  time  through  the  usual  processes  of  decay.  It 
needs  only  such  conditions  of  moisture  as  shall  prevent  the 
complete  decay  and  hence  favor  the  accumulation  of  the  organic 
matter,  to  give  us  beds  of  peat  and  ultimately  of  coal.  Plants 
of  the  type  of  sphagnous  mosses,  growing  continuously  above 

1  Geology  of  North  Carolina,  Vol.  I,  1875. 

2  Agricultural  Science,  Vol.  VII,  No.  3,  1893,  pp.  106-120. 


SWAMP  DEPOSITS  317 

and  dying  beneath,  hold  in  their  mass  sufficient  moisture  to 
exclude  atmospheric  air,  and  thus  themselves  bring  about  the 
proper  conditions  for  bog  making.  In  virtue  of  this  property 
such  may  gradually  rise  above  the  level  of  the  surrounding 
country,  as  is  the  case  with  the  Great  Dismal  Swamp  of  Vir- 
ginia and  numerous  others  that  need  not  be  mentioned  here. 
Instances  are  on  record  where  bogs  of  this  nature  have  grown 
so  far  above  the  natural  level,  that  during  seasons  of  unusual 
rainfall  they  have  burst,  and  flooded  adjacent  regions,  with  dis- 
astrous results.  The  rate  of  growth  of  such  accumulations  is 
naturally  quite  variable.  H.  S.  Gesner,  as  quoted  by  T.  Rupert 
Jones,1  states  that  in  Bavarian  moors  the  observed  increase  in 
peat,  in  forty-five  years,  amounted  to  from  2  to  3  feet  in  thick- 
ness ;  in  Oldenberg,  in  one  hundred  years,  to  4  feet ;  in  Ham- 
melsmoor,  Denmark,  to  2J  feet ;  and  in  Alpine  districts  to  4  and 
5  feet  in  from  thirty  to  fifty  years. 

The  peat  bogs,  so  characteristic  of  Ireland,  Scotland,  and 
other  northern  latitudes,  are  of  this  type.  A  section  of  the 
well-known  Bog  of  Allen,  made  in  county  Kildare,  is  given 
below.2 

THICKNESS 

(1)  Dark  reddish  brown ;  mass  compact ;  no  fibres  of  moss  visible  ; 

surface  decomposed  by  atmosphere 2    feet 

(2)  Light  reddish  brown  ;  fibres  of  moss  very  perfect 3      " 

(3)  Pale  yellowish  brown  ;  fibres  of  moss  very  perceptible 5      " 

(4)  Deep  reddish  brown  ;  fibres  of  moss  perceptible 8|    " 

(5)  Blackish  brown ;    fibres  of  moss  scarcely  perceptible,  contains 

numerous  twigs  and  small  branches  of  birch,  elder,  and  fir    .     3      " 

(6)  Dull  yellow-brown ;  fibres  not  visible  ;  contains  much  empyreu- 

matic  oil ;  mass  compact 3      " 

(7)  Blackish  brown  ;  mass  compact ;  fibres  not  visible  ;  contains  much 

empyreumatic  oil 10      " 

(8)  Black  mass,  very  compact ;  has  a  strong  resemblance  to  pitch  or 

coal ;  fracture  conchoidal  in  all  directions;  lustre  shining  ...    4      " 

Total  depth  of  bog 38£  feet 

Underlaid  by  3  feet  of  marl  containing  64  %  carbonate  of  lime,  4  feet  of  blue 
clay,  and  this  in  its  turn  by  clay  mixed  with  limestone  gravel  of  an  unknown 
thickness. 

1  Proc.  Geologists'  Association,  Vol.  VI,  No.  5,  January,  1880. 

2  T.  Rupert  Jones,  Proc.  of  the  Geologists'  Association,  London,  Vol.  VI,  No. 
5,  January,  1880.    This  authority  classifies  the  peat  bogs,  swamps,  and  marshes, 
as  follows  :  — 

I.  Peat  bogs  and  turf  moors  on  such  plateaux  as  flat  mountain  tops  and  wide 
hill  moors. 


318  THE   REGOLITH 

Deposits  of  the  cumulose  type  pass  by  all  gradations  into 
the  paludal,  swamp,  or  marsh  type  and  these  in  turn  into  ordi- 
nary alluvium.  Or  it  would  perhaps  be  better  to  reverse  this 
order,  since,  as  in  the  gradual  silting  up  of  an  enclosed  lake, 
we  may  have,  in  the  first  stages,  stratified  alluvium,  then  when 
the  waters  become  sufficiently  shallowed,  swamp  and  muck 
deposits,  and  lastly  the  deposits  of  pure  organic,  or  cumulose 
material. 

2.     TRANSPORTED  MATERIALS 

Because  of  the  constant  action  of  gravity,  the  well-known 
transporting  power  of  water,  the  wind  or  moving  ice,  few  re- 
sidual products  retain  for  any  length  of  time  their  virgin  purity, 
but  become  more  or  less  contaminated  with  materials  from  near 
or  distant  sources.  The  avalanches  of  mountain  regions  afford 
an  illustration  of  the  bodily  transfer  of,  it  may  be,  millions  of 
tons  of  matter  from  the  mountain  slopes  to  be  debouched  into 
the  valley  below  ;  the  slow-creeping  glacier  brings  down  its 
load  and  deposits  its  moraine  when,  succumbing  to  the  blan- 
dishments of  warmer  climes,  it  is  no  longer  able  to  bear  it  fur- 
ther :  spasmodic  winds  catch  up  the  smaller  particles  as  clouds 
of  dust  to  be  transported,  assorted,  and  redeposited  as  their 

II.  Peat  bogs  of  valleys  :  (1)  At  the  heads  of  valleys  ;  (2)  at  the  salient  angles 
within  river  curves ;  (3)  in  deserted  beds  of  rivers  ;  (4)  in  plains  and  lakes  of 
expanded  valleys  ;  (5)  special  peat  bogs  of  Denmark  and  the  black  earth  of  Rus- 
sia ;  (6)  river  deltas ;  (7)  maritime  peat  marshes,  where  certain  valleys  and  plains 
open  to  the  sea. 

Regarding  the  black  earth  of  Russia,  it  should  be  stated  that  this  is  now 
regarded  by  at  least  one  authority  (Hume,  Geol.  Mag.,  Vol.  I,  No.  2,  1894)  as 
being  but  a  local  phase  of  the  loess,  the  color  being  due  to  the  prevalence  of 
organic  matter. 

Shaler  (Ann.  Rep.  U.  S.  Geol.  Survey,  1888-89),  on  a  basis  of  physical  char- 
acters, classifies  the  inundated  lands  of  the  United  States  as  below  :  — 

c  . .  (  Grass  marshes. 

Marine  marshes  /Above  ^  tide  .  .    -  {  Mangrove  marshe, 

IT,,  . ,  •  f  Mud  banks. 

(Below  mean  tide  .  .     - 1 Eel_grass  areas. 

/„.  (Terrace. 

River  swamps   .     .     .     .{Estuarine 

T   ,  f  Lake  margins. 

Fresh-water  swamps.    .    .    >ake  swamps    .     .    .    •  j  Quaking  b&ogs. 

f  Wet  woods. 
Upland  swamps    .     .     •  j  Climbing  bogs. 

vAblation  swamps. 


COLLUVIAL   DEPOSITS 


319 


force  is  spent.  It  is,  however,  through  the  continual  transpor- 
tation of  running  streams,  both  in  the  past  and  present,  and 
through  the  action  of  moving  ice  in  ages  gone,  that  have  been 
brought  about  the  great  amount  of  transportation  and  admixt- 
ure characteristic  of  that  part  of  the  regolith  comprised  under 
the  general  name  of  drift.  According  to  which  of  the  agencies 
enumerated  prevailed,  we  may  subdivide  our  subject  as  folloAvs  : 
(1)  Colluvial  deposits,  (2)  alluvial  deposits,  (3)  seolian  de- 
posits, and  (4)  glacial  deposits,  though  as  we  proceed  we  shall 
find  that  the  lines  of  separation  are  not  in  all  cases  sharply 
drawn,  and  in  many  an  area  the  regolith  bears  impress  of  com- 
pounded agencies. 

(1)  Colluvial  Deposits.1  —  Under  this  head  it  is  proposed  to 
include  those  heterogeneous  aggregates  of  rock  detritus  com- 
monly designated  as  talus  and  cliff  debris.  The  material  of 
avalanches  may  also  be  classed  here.  Such  result 
wholly  from  the  transporting  action  of  gravity.  The 
deposits  in  themselves  are  comparatively  limited 
in  extent,  ever  varying  in  composition,  and  are 
composed  of  an  indiscriminate  admixture  of 
N^  particles  of  all  sizes,  from  those  as  fine  as 
dust  to  blocks  it  may  be  of  hundreds  of 
tons'  weight.  Such  are  necessarily  limited 
to  the  immediate  vicinity  of  the  cliffs  or 
mountains  from  which  they  are  derived. 
As  loosened  by  heat  or  frost  from  the 


FIG.  31.  — Diagram 
showing  the  history 
of  a  talus,      a,  bed 
rock;   bb,  talus;  c,    de- 
stroyed portion  of  a  cliff, 
the   material  being  now  in 
the  talus. 


So&tearlng ponton, 
"talus 


parent  masses,  the  fragments 

tumble  down  the  slopes,  gradually 

accumulating  in  beds  the  slope  of  which  is  limited  only  by  the 

laws  of  gravity  and  the  character  of  the  debris.     (See  PI.  23.) 

Inclinations  of  30°  are  common  ;   less  commonly  of  40°.     From 

1  From  the  Latin  "colluvies,"  a  mixture.     The  term  as  here  used  is  more 
restricted  in  its  meaning  than  as  used  by  Professor  Ililgard. 


320 


THE   REGOLITH 


their  mode  of  origin  it  is  natural  that  the  individual  particles 
should  be  mainly  angular  and  comparatively  fresh.  In  fact, 
they  represent  rock-weathering  through  disintegration,  and 
not  decomposition,  which  will  come  later.  Below,  i.e.  further 
down  the  slopes  and  in  the  edges  of  the  valleys,  these  coarse, 
illy  assorted  deposits  pass  gradually  into  soils ;  above,  they 
consist  simply  of  masses  of  loose  rock  wholly  unfitted  for  the 
support  of  vegetable  life.  (Fig.  31.)  Through  becoming  sat- 
urated with  water,  ice,  or  snow,  such  at  times  become  loosened 
from  the  steep  slopes  on  which  they  lie  and  slide  down  in  the 
form  of  avalanches  into  the  valleys.  Although  comparatively 
limited  in  their  extent,  these  latter,  owing  to  the  resistless 
energy  and  suddenness  of  their  advance,  are  sometimes  appall- 
ingly destructive,  as  has  been  repeatedly  illustrated  in  the  Swiss 
Alps,  and  other  mountain  regions.  The  geographic  distribution 
of  talus  deposits  as  controlled  by  climatic  conditions  has  been 
already  noted  (p.  283). 

(2)  Alluvial  Deposits.  —  The  deposits  included  under  this 
head  differ  structurally  from  those  thus  far  described  in  that 
they  are  always  more  or  less  distinctly  stratified,  or  bedded. 
In  writing  of  the  formation  of  sedimentary  rocks,  and  again 
when  treating  of  the  action  of  running  water,  a  few  figures 
were  given  relative  to  the  amount  of  transported  debris  de- 
posited yearly  in  the  Gulf  of  Mexico.  In  a  similar  way  the 
amount  of  debris  carried  annually  to  the  ocean  by  some  of  the 
chief  rivers  of  the  world  has  been  estimated  as  below  :  — 


CUBIC   FEET 

Mississippi 7,468,694,400 

Upper  Ganges       .     .     .    6,368,077,440 
Hoang-Ho 17,520,000,000 


CUBIC   FEET 

Rhone 600,000,800 

Danube 1,253,738,600 

Po 1,510,147,000 


The  muddy  condition  of  the  water,  caused  by  this  sus- 
pended matter,  is  so  conspicuous  a  feature  of  certain  rivers  that 
they  have  received  special  names  on  this  account.  Hoang-Ho 
means  simply  yellow  river ;  Missouri  is  the  Indian  name 
for  Big  Muddy ;  while  the  famous  Red  River  of  the  North  is 
so  called  merely  because  of  the  red  mud  it  carries.  Such  silt- 
bearing  streams,  .flowing  into  lakes  and  tideless  seas,  begin 
depositing  their  loads  so  soon  as  their  currents  are  checked, 
building  up  thus  the  so-called  delta  deposits  for  which  the 
Mississippi,  the  Po,  Ganges,  and  the  Nile  are  noted. 

The  character  of  the  material  in  the  delta  deposits  is  vari- 


ALLUVIAL   DEPOSITS 


321 


able  only  within  certain  limits,  consisting  always  of  siliceous 
sand  and  mud  intermingled  with  organic  matter. 

Professor  Judd,  who  examined  samples  from  borings  in  the 
alluvial  deposits  of  the  Nile  delta,  found  the  materials  to  vary 
abruptly  in  texture  from  the  surface  downward,  the  variations 
following  no  recognizable  law.  The  percentage  amounts  of 
constituents  classed  as  sand  and  mud,  as  obtained  from  (I) 
borings  at  Kasr-el-Nil,  Cairo,  (II)  Kafr-ez-Zayat,  and  (III) 
Tantah,  are  given  in  the  table  below. 


in 


DEPTH 

SAND 

MUD 

SAND 

MUD 

SAND 

MUD 

QI 

10 

% 

% 

% 

% 

3'0"     .     .    . 

.... 

.... 

2.35 

97.65 

4'0"     .     .     . 

.... 

30.42 

69.58 

1.71 

98.29 

6'  0"     ... 

5.77 

94.33 

.... 

8'C"     .     .     . 

7.27 

92.73 

11'  0"     .     .     . 

.... 

50.99 

49.01 

.... 

10'  0"     ... 

86.27 

13.73 

.... 

.... 

.... 

17'  6"     .     .     . 

79.65 

20.35 

.... 

.... 

.... 

18'  0"     ... 

.... 

.... 

.... 

.... 

8.78 

91.22 

19'  0"     .     .     . 

.... 

.... 

87.41 

12.59 

.... 

22'  6"     .     .     . 

.... 

.... 

.... 

31.16 

68.44 

20'  0" 

90.19 

9.81 

31'  0"     ... 

.... 

..,. 

39.43 

60.57 

35'  0"     ... 

.... 

86.42 

13.58 

.... 

38'  0"     ... 

65.05 

34.95 





.... 

40'  0'      .     .     . 

.... 

.... 

81.94 

18.06 

80.70 

19.30 

40'  6'      ... 

80.83 

19.17 

.... 

.... 

.... 

45'  0'      ... 

68.72 

31.28 

.... 

.... 

.... 

40'  0'      .     .     . 

.... 

.... 

.... 

95.90 

4.10 

48'  0'      ... 

.... 

87.23 

12.77 

.... 

.... 

55'  0'      ... 

.... 



0.25 

99.75 

97.71 

.... 

50'  0'      .     .     . 

.... 

.... 

99.53 

2.29 

58'  0'      ... 

.... 

.... 



.... 

59.09 

0.47 

60'  0'      ... 

.... 



12.60 

87.40 

.... 

40.91 

66'  0'      .     .     . 

.... 

62.07 

37.93 

.... 

68'  0"     ... 

.... 

7.76 

.... 

73'  0"     ... 





.... 

59.95 

92.24 

75'  0"     .     .     . 

.... 

66.38 

36.62 

.... 

40.05 

The  material  described  as  sand  consists  of  rounded,  angular, 
and  sub-angular  grains.  The  well-rounded  granules  are  mainly 
of  quartz  and  feldspar  ;  the  angular  and  sub-angular  of  quartz, 
feldspars,  hornblende,  and  augite,  with  smaller  quantities  of 
mica,  tourmaline,  sphene,  iolite,  zircon,  fluor-spar,  and  magnetite 


322  THE   REGOLITH 

all  in  a  nearly  unaltered  condition.  The  feldspars  are  mainly 
orthoclase  and  microcline  —  rarely  a  soda-lime  variety  —  and 
in  a  state  of  surprising  freshness.  The  quartz  is  in  part  the 
quartz  of  granitic  rocks  and  the  larger  grains  well  rounded, 
best  described  as  microscopic  pebbles.  He  says  :  "  It  is  evi- 
dent that  these  sand  grains  have  been  formed  by  the  breaking 
up  of  granitic  and  metamorphic  rocks,  or  of  older  sandstones 
derived  directly  from  such  rocks.  The  larger  grains  exhibit 
the  perfect  rounding  and  polishing  now  recognized  as  charac- 
teristic of  yeolian  action ;  the  smaller  ones  from  their  larger 
surfaces  in  proportion  to  their  weight,  have  undergone  far  less 
attrition  in  their  passage  through  the  air ;  but  it  is  fair  to  con- 
clude that  they  are  really  desert  sand,  derived  from  the  vast 
tracts  which  lie  on  either  side  of  the  Nile  valley,  and  swept 
into  it  by  the  action  of  the  wind."  The  material  described 
as  mud  is  composed  of  essentially  the  same  materials  as  the 
sands,  but  in  a  more  finely  divided  state.  There  is  an  entire 
absence  of  anything  like  kaolin,  though  there  are  present 
particles  of  organic  matter  and  frustules  of  diatoms.  The 
surprising  freshness  of  the  materials  and  lack  of  kaolin  is 
regarded  as  indicative  of  an  origin  through  the  action  of  heat 
and  frost ;  i.e.  through  mechanical  agencies  rather  than  through 
the  processes  of  rock  decomposition.1 

But,  as  has  been  already  noted,  only  a  part  of  the  sediment 
carried  by  any  stream  reaches  its  mouth.  A  comparatively 
small,  but,  from  our  present  standpoint,  very  important  portion 
is  carried  during  seasons  of  high  water  beyond  the  usual  chan- 
nels and  spread  out  over  the  flood  plains,  as  described  on  p.  287. 
Such  deposits  are,  as  a  rule,  plainly  stratified,  and  consist  of 
mineral  matter  in  a  finely  comminuted  condition  derived,  it  may 
be,  from  the  breaking  down  of  a  great  variety  of  rocks.  Their 
physical  and  chemical  properties,  as  well  as  the  periodic  char- 
acter of  their  deposition,  are  favorable  to  the  formation  of  soils 
possessing  great  strength  and  fertility.  Both  fertility  and  rate 
of  deposition  in  such  cases  are  augmented  through  plant  growth, 
which  takes  place  with  great  rapidity  wherever  climatic  condi- 
tions are  favorable.  So  soon  as  the  water  leaves  the  flood  plain, 
a  host  of  moisture-loving  plants,  as  reeds  and  rushes,  spring  up 
in  countless  numbers  to  die  down  again  in  the  fall,  and  yield 
the  carbon  and  nitrogeneous  constituents  to  serve  as  fertilizers, 
1  Proc.  Royal  Soc.  of  London,  Vol.  XXXIX,  1885,  p.  213. 


ALLUVIAL   DEPOSITS 


323 


and  augment  the  crop  of  the  following  year.  Moreover,  the 
remaining  stems  and  fallen  leaves  of  the  plants  serve  to  retard 
the  running  waters  of  each  succeeding  flood,  catching  in  their 
meshes  the  floating  sediments  which  might  otherwise  be  carried 
seaward.  The  Anacostia,  which  empties  into  the  Potomac  River 
east  of  Washington,  serves  as  a  good  illustration  of  the  working 
of  these  agencies.  A  century  ago  the  stream  was  navigable  by 
coasting  crafts  as  far  as  Bladensburg.  Now,  owing  to  shallow 
waters,  nothing  but  rowboats  can  navigate  beyond  the  Navy 
Yard  at  Washington.  Each  season  the  stream,  murky  with 
suspended  silt  from  cultivated  fields  along  its  shores,  comes 
down,  till,  ponded  back  by  tides,  it  begins  to  deposit  its  load. 
As  year  by  year  its  bed  was  thus  raised,  water  plants,  encroach- 
ing more  and  more  from  shallow  shores,  still  further  dammed 


FIG.  32. 


its  sluggish  current  till  now,  during  summer  months,  it  is  little 
more  than  a  stagnant  pond  full  of  rank  vegetation,  and  a  source 
of  odors  foul  and  atmospheres  enervating.  The  so-called 
"  Potomac  Flats  "  south  of  the  city  of  Washington  owed  their 
origin  and  unhealthy  conditions  to  similar  processes. 

The  method  of  alluvial  deposition  in  the  flood  plain,  or 
delta,  of  the  lower  Mississippi  has  been  worked  out  by  McGee,1 
from  whom  we  cannot  do  better  than  quote  in  considerable 
detail. 

In  length  this  flood  plain  reaches  from  the  mouth  of  the  Ohio 
1100  miles  measured  along  the  river,  or  half  as  far  measured 
in  an  air  line,  to  the  Gulf,  and  is  bounded  on  the  east  by  the 
bluff  rampart  separating  it  from  the  contiguous  district ;  it  is 
bounded  on  the  west  by  a  less  continuous  and  less  conspicuous 
rampart  crossing  the  Arkansas  River  at  Little  Rock  and  grad- 
ually failing  southward  until  this  district  and  its  more  westerly 

1  The  Lafayette  Formation,  Ann.  Rep.  U.  S.  Geol.  Survey,  1890-91. 


324  THE   REGOLITH 

neighbor  nearly  blend.  The  surface  of  this  otherwise  monoto- 
nous district  is  relieved  by  a  few  small  tracts  of  higher  land. 
Most  conspicuous  of  these  is  Crowley  Ridge  in  eastern  Arkansas, 
a  long  belt  of  upland  stretching  from  the  southeastern  Missouri 
southward  between  the  White  and  St.  Francis  rivers  to  the 
Mississippi  at  Helena.  This  belt  of  upland  rises  100  or  200  feet 
above  the  insulating  flood  plain,  and  in  its  steepness  of  slope 
and  rugosity  of  outline  fairly  simulates  the  eastern  rampart 
overlooking  the  "  delta  "  in  corresponding  latitudes. 

The  vast  lowland  tract  comprised  in  and  constituting  most  of 
this  district  is  at  once  the  most  extensive  and  most  complete 
example  of  a  land  surface  lying  at  base-level  or  a  trifle  below 
that  the  continent  affords. 

It  is  trenched  longitudinally  by  the  Mississippi,  and  trans- 
versely by  the  White,  Arkansas,  Red,  and  other  large  rivers ; 
between  these  greater  waterways  it  is  cut  into  a  labyrinth  of 
peninsulas  and  islands  by  a  network  of  lesser  tributaries  and 
distributaries,  the  former  gathering  the  waters  from  its  own 
surface  and  from  adjacent  country,  and  the  latter  aiding  the 
main  river  to  discharge  its  vast  volume  of  water  and  its  immense 
load  of  detritus  into  the  Gulf.  The  whole  surface  lies  so  low 
that  it  is  flooded  by  periodic  overflows  of  the  Mississippi  and 
its  larger  tributaries,  and  with  each  flood  receives  a  fresh  coat- 
ing of  river  sediment ;  and  much  of  the  flood  plain,  fertilized  by 
freshet  deposits,  is  clothed  with  luxuriant  forests  and  dense 
tangles  of  undergrowth,  or  with  brakes  of  cane,  or  with  sub- 
tropical shrubbery,  only  a  few  of  the  broader  inter-stream  tracts 
being  grassed.  Partly  by  reason  of  this  mantle  of  vegetation, 
the  current  of  each  overflow  is  checked  as  the  river  rises  above 
its  banks,  and  most  of  the  sediment  is  dropped  near  by;  and  so 
the  Mississippi,  the  White,  the  Arkansas,  and  the  Red,  as  well 
as  each  lesser  tributary  and  each  distributary  from  the  great 
Atchafalaya  down,  are  flanked  by  natural  levees  of  height  and 
breadth  proportionate  to  the  depth  and  breadth  of  the  stream. 
The  network  of  waterways  is  thus  a  network  of  double  ridges 
with  channels  between;  and  each  inter-stream  area  is  virtually 
a  shallow,  dish-like  pond  in  which  the  waters  of  the  floods  lie 
long,  to  be  drained  finally,  perhaps,  through  fresh-made  breaks 
in  the  natural  dikes,  weeks  after  the  stream  flood  subsides.  In 
the  southern  part  of  the  district  the  inter-stream  basins  approach 
tide  level  and  drain  still  more  slowly ;  in  the  sub-coastal  zone 


ALLUVIAL    DEPOSITS 


325 


many  of  the  basins  are  permanent  tidal  marshes.  In  the  western 
part  of  the  district  is  an  area  in  which  the  inter-stream  basins 
lie  so  high  that  they  are  invaded  only  by  the  highest  floods  and 
veneered  with  only  the  finest  sediments ;  in  some  cases  these 
sediments  are  so  fine  and  so  compactly  aggregated  and  the 
surface  is  so  ill  drained  and  watered  that  trees  may  hardly 
take  root,  and  these  are  either  drowned  by  the  floods  or  with- 
ered by  the  sun.  in  the  drought.  Such  portions  of  the  sur- 
face are  but  scantily  covered  with  coarse  grass  and  form 
the  "black  prairies"  of  southern  Arkansas  and  northwestern 
Louisiana. 

It  is  to  just  such  processes  as  those  described  that  the  Nile 
valley  owes  its  remarkable  fertility.  The  sediments  depos- 
ited over  these  plains  during  the  season  of  freshets  consist  of 
fine  sand  brought  down  by  the  Blue  Nile  and  the  Atbara  from 
the  decomposing  siliceous  rocks  of  mountainous  Abyssinia.  The 
gneisses  and  granites  yield  their  detritus  to  the  lixiviating  in- 
fluence of  the  mountain  torrents  and  majestic  Nile,  the  clayey 
particles  being  borne  seaward,  while  the  fresh  quartzose,  feld- 
spathic  and  other  siliceous  particles,  and  smaller  traces  of  apa- 
tite and  alkaline  carbonates  remain  in  just  the  right  stage  of 
subdivision  to  yield  a  soil,  which  has  brought  forth  for  a  period 
of  over  4000  years  crop  after  crop  without  artificial  fertilization. 

The  following  table  will  serve  to  show  the  physical  character- 
istics of  alluvial  deposits,  a  portion  of  which  are  but  reasserted 
materials  from  the  glacial  drift. 

APPROXIMATE  NUMBER  OF  GRAINS  OF  SAND,  SILT,  AND  CLAY  IN  ONE  GRAMME 
OF  ALLUVIAL  SUB-SOIL  FROM  ILLINOIS 


DIAMETER 

CONVENTIONAL 

(a) 

(6) 

(c) 

MM. 

NAMES 

CHILLICOTHE 

EOCKFOBD 

AMERICAN  BOTTOMS 

2-1 

Fine  gravel 

0 

1 

0 

1-.5 

Coarse  sand    . 

83 

48 

0 

.5-.25 

Medium  .     .     . 

6,755 

3,428 

5 

.25-.! 

Fine  sand  .     . 

18,660 

29,300 

194 

.1-.05 

Very  fine  sand 

53,470 

212,400 

151,400 

.05-.01 

Silt     .... 

4,670,000 

5,888,000 

12,230,000 

.01-.005 

Fine  silt      .     . 

86,860,000 

115,100,000 

195,600,000 

.005-.0001 

Clay  .... 

2,537,000,000 

3,842,000,000 

14,680,000,000 

Total.     .     .     . 

2,628,608,968 

3,963,232,177 

14,887,981,599 

(a)  Terrace  of  Glacial  age.     (6)  Flood  deposits, 
(bottom  land  of  Mississippi) . 


(c)  Post-glacial  terrace 


326  THE   REGOLITH 

The  same  processes  active  in  delta  formation  are  manifested 
on  a  smaller  scale  in  the  gradual  silting  up  of  many  inland 
lakes,  particularly  those  of  glacial  origin,  the  rapidity  of  the 
filling  being  augmented  by  aquatic  plants. 

These  lakes  lie  not  infrequently  between  high  hills,  being 
fed  by  one  or  more  streams  flowing  through  narrow  valleys, 
and  having  outlets  at  the  opposite  extremity.  Soon  after  the 
close  of  the  Glacial  epoch,  we  may  imagine  one  of  these  to 
have  existed  as  a  lake  of  clear  blue  water  of  varying  depths, 
filled  with  abundant  fish  and  wild  fowl.  But  the  little  streams 
which  fed  it  brought  down  continually  sand  and  silt  to  be  de- 
posited at  varying  distances  so  soon  as  the  currents  fall  to  sleep 
within  the  bosom  of  the  lake.  Hence  each  year  it  shallows, 
and  the  pure  white  water-lily,  reeds,  and  the  rotting  trunks  of 
trees  and  shrubs  encroach  upon  its  shores  until  in  course  of 
time  there  remains  but  a  flat  plain,  for  a  time  subject  to  annual 
inundations,  but  ultimately  permanently  above  the  level  of  but 
the  most  severe  floods,  and  through  which  flow  in  a  meander- 
ing course  the  sluggish  streams  that  first  gave  it  birth  and  then 
wrought  its  extinction.  This  is  the  story  of  thousands  of  the 
so-called  meadows,  swales,  swamps,  and  intervals  throughout 
the  northern  portion  of  the  United  States,  and  the  process  in 
some  easily  recognizable  stage  may  be  found  in  almost  any  lake 
or  pond  now  remaining. 

It  is  a  striking  thought  that  all  our  lakes  are  but  transient 
enlargements  of  pre-existing  streams,  and  will  in  time,  perhaps 
even  before  our  own  species  is  extinct,  become  converted  into 
broad  expanses  of  meadow  lands ;  and  that  our  children's  chil- 
dren may  yet  sow  and  reap  from  rich  and  fertile  areas  which 
now  echo  only  to  the  cry  of  water-fowls,  and  whose  blue  ex- 
panse is  broken  but  by  wind-born  waves  and  leaping  fish. 

The  lithological  character  of  the  deposits  thus  formed  vary 
within  certain  limits  almost  indefinitely,  since  everything  de- 
pends on  the  character  and  quantity  of  the  silt  brought  down 
by  the  streams.  Rarely,  if  ever,  are  they  clayey,  since  the  finer 
particles  are  carried  beyond.  In  nearly  all  instances  they  are 
found  to  consist  of  very  fine  sand,  largely  siliceous,  permeated, 
often  quite  blackened,  through  the  presence  of  organic  matter. 
Such  are  the  mucks  or  mucky  soils  of  New  England. 

So  abundant  is  this  organic  matter  that,  when  dried,  such  are 
not  infrequently  used  locally  for  mulching  purposes,  though 


ALLUVIAL   DEPOSITS  327 

in  their  fresh  condition  they  are  sour  and  almost  worthless 
except  for  growing  sedges  and  the  ranker  kinds  of  forage 
grass.  During  the  later  stages  of  the  process  of  filling  up, 
deposition  of  sediments  may  almost  entirely  cease,  since  the 
water  no  longer  rises  above  the  level  of  past  accumulations. 
In  such  cases  the  final  stages  consist  simply  in  the  accumu- 
lation of  organic  matter  and  the  deposits  come  to  closely 
resemble,  or  are  even  superficially  identical  with,  the  cumulose 
deposits  already  described.  This  same  statement  holds  good 
also  for  the  closely  related  salt-water  marsh  or  paludal  deposits, 
to  be  noted  later. 

Loess  and  Adobe. — Under  the  head  of  transported  deposits, 
we  must  also  consider  the  so-called  loess  of  the  Mississippi  val- 
ley in  our  own  country ;  of  the  Rhine  valley,  and  other  parts 
of  Europe  ;  of  northern  China  and  the  Russian  steppes,  though, 
as  we  shall  see,  the  name  includes  deposits  which,  while  having 
many  physical  properties  in  common,  may  vary  widely  in  com- 
position as  well  as  in  method  of  deposition.  It  is  more  than 
doubtful,  indeed,  if  the  name,  through  misapprehension,  has  not 
been  so  loosely  applied  as  to  rob  it  of  its  proper  geological 
significance. 

The  loess  of  China,  made  famous  through  the  researches  of 
Richtofen,  is  now  regarded  by  some  authorities 1  as  of  the 
same  nature  as  our  adobe.  Richtofen  himself,  it  will  be  re- 
membered, regarded  the  Chinese  loess  as  largely  an  yeolian 
deposit,  as  due  to  the  action  of  wind  in  transporting  for  long 
distances  the  fine  detritus  swept  by  rain  and  wind  from  moun- 
tain slopes  into  enclosed  basins,  to  ultimately  become  entangled 
and  deposited  among  the  growing  vegetation.  This  foreign 
material,  intermingled  with  the  collective  residue  of  herba- 
ceous plants,  with  the  inorganic  residuum  from  the  decay  of 
prairie  vegetation  for  countless  generations,  makes  up  its  mass 
over  many  hundreds  of  square  miles  of  territory,  and  to  depths 
in  places  of  thousands  of  feet.  The  characteristics  of  the  loess, 
as  found  in  China,  are  those  of  a  fine  calcareous  silt  or  clay,  of 
a  yellowish  or  buff  color,  so  slightly  coherent  that  it  may  be 
readily  reduced  to  powder  between  the  thumb  and  fingers,  and 
yet  possessing  such  tenacity  as  to  resist  the  ordinary  weather- 
ing action  of  the  atmosphere,  and,  wherever  cut  by  stream 
erosion  or  other  means,  to  stand  with  vertical  walls,  even 
1  See  I.  C.  Russell,  Subaerial  Deposits  of  North  America,  Geol.  Mag.,  August,  1889. 


328  THE   REGOLITH 

though  they  may  be  hundreds  of  feet  in  height.  The  loess 
country  is  described  as  thus  cut  up  by  an  almost  impassable 
system  of  gorges,  so  that  to  cross  it  in  any  fixed  direction  is 
almost  an  impossibility.  "Wide  chasms  are  surrounded  by 
castles,  towers,  peaks,  and  needles,  all  made  up  of  yellow  earth, 
between  which  gorges  and  chasms  radiate  labyrinthically  up- 
wards into  the  walls  of  solid  ground  around.  High  upon  a 
rock  of  earth — steeper  than  any  rock  of  stone  —  stands  the 
temple  of  the  village,  or  a  small  fortress  which  affords  the 
villagers  a  safe  retreat  in  times  of  danger.  The  only  access 
to  such  a  place  is  by  a  spiral  stairway  dug  out  within  the  mass 
of  the  bluff  itself.  In  this  yellow  defile  there  are  innumerable 
nooks  and  recesses,  often  enlivened  by  thousands  of  people, 
who  dwell  in  caves  dug  in  the  loess."1 

One  of  the  striking  features  of  the  loess,  both  in  China  and 
elsewhere,  is  the  abundance  of  minute  tubes  or  canals  —  lined 
with  carbonate  of  lime  —  which  traverse  it  from  above  down- 
ward, and  which  are  assumed  by  some  to  be  due  to  root  fibres. 
It  is  the  presence  of  these  presumably  that  causes  the  vertical 
cleavage,  and  at  the  same  time  the  remarkable  absorptive  quali- 
ties for  which  the  loess  is  noted.  Such  is  the  material  which 
for  more  than  three  thousand  years  has  brought  forth  crops 
continuously,  and  without  exhaustion,  over  many  square  miles 
of  the  Chinese  Empire.  Its  distribution  in  Europe  is  given  as 
extending  from  the  French  coast  at  Sangatte,  eastward  across 
the  north  of  France  and  Belgium,  filling  up  the  depressions  of 
the  Ardennes,  passing  far  up  the  valleys  of  the  Khine  and  its 
tributaries,  the  Neckar,  Main,  and  Lahr ;  likewise  those  of  the 
Elke  above  Meissen,  the  Weser,  Mulde,  and  Saale,  the  upper 
Oder  and  Vistula.  Spreading  across  upper  Silesia,  it  sweeps 
eastward  over  the  plains  of  Poland  and  southern  Russia,  where 
it  forms  the  substratum  of  the  tschernoseun,  or  black  earth.  • 
It  extends  into  Bohemia,  Moravia,  Hungaria,  Galicia,  Transyl- 
vania, and  Roumania  far  up  into  the  Carpathians,  where  it 
reaches  heights  of  from  2000  to  5000  feet  above  sea-level.  In 
northern  China  it  spreads  over  a  large  portion  of  the  region 
drained  by  the  Hoang-Ho.  For  nearly  a  thousand  miles 
from  the  borders  of  the  great  alluvial  plain  of  Pechele,  through 
the  provinces  of  Shan  si,  Shensi,  and  Kansu,  everywhere  to  the 

JThe    Chinese    Loess    Puzzle,   by  J.   D.    Whitney,   American    Naturalist, 
December,  1877. 


LOESS   AND   ADOBE 


329 


northern  base  of  the  range  of  the  Tsing-ling-shan,  the  loess 
may  be  followed  to  the  very  divide  which  separates  the  basin 
of  the  Hoang-Ho  from  the  region  destitute  of  drainage,  into 
the  sea.  Toward  the  north  it  reaches  almost  to  the  edge  of 
the  Mongolian  plateau.  The  entire  area  covered  continuously 
is  stated  to  be  as  large  as  the  whole  of  Germany,  while  it  is 
found  in  more  or  less  detached  portions  over  an  area  in  addi- 
tion, nearly  half  as  large.  In  the  United  States  the  loess 
covers  thousands  of  square  miles  throughout  the  drainage 
basin  of  the  Mississippi  River.  It  is  found  in  Ohio,  Indiana, 
Michigan,  Iowa,  Kansas,  Nebraska,  Illinois,  Tennessee,  Ala- 
bama, Mississippi,  Louisiana,  Arkansas,  Missouri,  Kentucky, 
and  the  Indian  Territory.  According  to  Professor  Aughey  it 
prevails  over  at  least  three-fourths  of  Nebraska,  to  a  depth 
ranging  from  5  to  150  feet,  and  furnishes  a  soil  of  extraor- 
dinary strength  and  fertility.  As  here  found,  however,  the 
aeolian  hypothesis  fails  to  satisfactorily  explain  all  the  exist- 
ing conditions,  arid  there  is  little  doubt  but  that  it  represents 
in  large  part  the  fine  silt,  the  glacial  flour  brought  down  by 
the  ice  of  the  Glacial  epoch,  borne  southward  by  streams,  and 
deposited  in  water  just  sufficiently  in  motion  to  carry  the  fine 
clay  farther  away.  The 
loess,  in  fact,  illustrates 
in  a  remarkable  manner 
the  wonderful  assorting 
power  of  water. 

Microscopic  and  chemi- 
cal examinations  of  loess 
sustain  this  hypothesis. 
The  particles  are  as  a  rule 
quite  fresh  and  sharply 
angular.  Out  of  150,000 
particles  examined  under 
the  microscope  only  about 
3%  measures  above  .0025 
of  a  millimetre  and  1  % 
over  .005  of  a  millimetre. 

f^  .  FIG.  33.  —  Showing  outlines  of  particles  in 

(Quartz     IS     the     prepon-  Chinese  loess. 

derating    material,   with 

lesser  amounts  of  orthoclase  and  plagioclase  feldspars,  white 

and  dark  micas,  hornblende,  augite,  magnetite,  dolomite,  and  cal- 


330 


THE    KEGOLITH 


BAD 
RLA 


O  t^ 

^  0 


1—1      .   i—  i         co   o   <r> 

1—1        .CCgtNCOO 


g 


i-H          rH          C<c 


C1OCM        '         '         '        '     CO     O     CO     £H     CO     O         '.         ',  CO 

g 

9   So  Ot-CNOOOOODTtir-iT-ir-iOOO 

'e 

w 
«a 

a" 

COCOCOCO(MCOOOi-iCOCOOOi-HOi 
CO     O     O     O     O     i— i     f"i     i^     C^     G^     O     O     O 

:  ;  g  ^  ;    :  :  :  ;  i" 

^  ^   o     C /-^    <D 

o  o  :s  §  o  3 

S  E  ^  ^   '  ^  B  -S 

ac^     *^  GJ 

^      --H         O       r^ 

Ii!l|!SltlP!i! 

II  If  Iff  IN  ft  111 


LOESS   AND   ADOBE 


331 


cite.  The  loess  of  the  Rhine  valley  and  of  China  offers  no  differ- 
ences that  can  be  readily  described,  though,  as  will  be  noticed 
by  reference  to  the  analyses,  there  may  be  a  wide  difference  in 
chemical  composition.  Indeed,  the  essential  characteristic  of 
the  loess  is  a  physical  rather  than  a  chemical  one,  and  it  is 
doubtless  to  this  that  is  due  its  uniform  fertility.  On  p.  330 
are  given  analyses  of  loess  from  the  United  States,  the  Rhine 
valley,  and  from  Switzerland. 

The  following  table  will  serve  to  show  the  fine  state  of  sub- 
division in  which  the  particles  exist  in  loess  as  well  as  in  the 
dust  brought  down  by  snow,  which  will  be  described  on  p.  344. 


I 

n 

III 

IV 

CONSTITUENTS 

it 

H?0 
g|l 

IP 

KIVER  LOESS  : 
VIRGINIA  CITY, 
ILLINOIS 

LOESS  : 
NEBRASKA 

DUST  FROM  SNOW  : 
ROCKVILLE, 
INDIANA 

Moisture          

5.40% 

3.17% 

Organic  matter    

4.96 

11.98 

0.00  % 

0.00  % 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.00 

0.01 

0.00 

0.00 

Fine  sand. 

0.01 

0.10 

0.00 

0.00 

Very  fine  sand 

7.68 

24.84 

23.14 

0.00 

Silt 

61.85 

60.98 

54.81 

69.37 

Fine  silt                                   .... 

9.60 

2.80 

2.46 

5.80 

Clay 

15.15 

6.15 

9.45 

9.68 

Aughey l  gives  the  following  section  of  the  loess  and  soil  in 

Nebraska. 

(1)  Loess 4    feet 

(2)  Black  soil 2 

(3)  Loess 4 

(4)  Black  soil 1 

(5)  Loess 6 

(6)  Black  soil 

(7)  Stratified  loess 15 

This  alternation  is  accounted  for  on  the  assumption  of  fre- 
quent changes  of  level  during  the  loess-forming  period.  It 
would  seem  that  the  loess  was  deposited  in  shallow  water  and 
that  as  the  lake  became  filled  plant  life  came  in  as  in  modern 

1  Physical  Geology  and  Geography  of  Nebraska,  p.  276. 


332  THE    REGOLITH 

bogs  and  marshes,  and  throve  until  sufficient  organic  matter. was 
formed  to  make  the  black  soil  layer.  A  period  of  subsidence 
followed,  more  loess  was  deposited  and  the  previous  condition 
repeated,  this  process  going  on  till  all  the  layers  were  formed. 
The  material  of  the  loess,  in  this  case,  would  seem  most  likely 
to  have  been  of  seolian  origin. 

The  name  adobe  is  given  to  a  calcareous  clay  of  a  gray-brown 
or  yellowish  color,  very  fine-grained  and  porous,  which  is  suffi- 
ciently friable  to  crumble  readily  in  the  fingers,  and  yet,  like 
loess,  has  sufficient  coherency  to  stand  for  many  years  in  the 
form  of  vertical  escarpments,  without  appreciable  talus  slopes. 
The  material  of  the  adobe  is  derived  from  the  waste  of  the 
surrounding  mountain  slopes,  the  disintegration  being  largely 
mechanical.  According  to  Prof.  I.  C.  Russell,1  from  whose  de- 
scriptions is  drawn  a  portion  of  what  is  given  here,  it  is  assorted 
and  spread  out  over  the  valley  bottom  by  the  action  of  ephemeral 
streams,  where  it  becomes  mixed  with  dust  blown  by  the  winds 
from  the  neighboring  mountains,  and  rendered  more  or  less 
coherent  by  the  cementing  action  of  interstitial  carbonate  of  lime. 

Hilgard  2  limits  the  name  adobe  to  the  distinctly  clayey  soils  of 
the  arid  regions,  and  divides  them  into  two  classes,  —  the  upland 
and  the  valley  adobes,  the  first  being  derived  mainly  from  the 
disintegration,  in  place,  of  clay  shales,  while  the  second  are 
mostly  paludal  or  swamp  formations,  and  represent  either  the 
finest  materials  that  remain  suspended  in  slack  water,  from  any 
source,  or  sometimes  the  direct  washings  of  the  clayey  soils  of 
the  hills.  Whichever  authority  we  follow,  it  is  evident  the 
name  includes  materials  alike  not  in  mode  of  origin  or  com- 
position, but  only  in  physical  characteristics. 

Adobe  forms  the  soil  of  a  large  portion  of  the  rainless  region 
of  the  United  States.  It  is  found  therefore  in  Colorado,  Utah, 
Nevada,  southern  California,  Arizona,  New  Mexico,  and  west- 
ern Texas,  as  well  as  in  the  southern  portion  of  Idaho,  Wyoming, 
and  Oregon.  It  has  also  a  wide  distribution  in  Mexico.  In 
the  United  States  it  occurs  from  near  the  sea-level  in  Arizona, 
and  even  below  it  in  southern  California  up  to  an  elevation  of 
at  least  6000  or  8000  feet  along  the  eastern  border  of  the  Rocky 
Mountains,  and  in  the  elevated  valleys  of  New  Mexico,  Colorado, 
and  Wyoming. 

1  Subaerial  Deposits  of  North  America,  Geol.  Mag.,  August,  1889. 

2  Bull.  3,  U.  S.  Weather  Bureau,  Dept.  of  Agriculture,  1892. 


LOESS   AND   ADOBE  333 

The  maximum  thickness  of  the  various  deposits  grouped 
under  this  name  is  not  in  all  cases  readily  determined,  for  the 
reason  that  it  is  still  accumulating  and  has  not  been  sufficiently 
dissected  by  erosion  to  expose  sections  to  any  considerable 
depth.  Many  of  the  valleys  of  the  arid  region  have  been  filled 
by  it  to  a  depth  of  2000  or  3000  feet.  In  the  larger  valleys 
there  are  rocky  crests,  called  "  lost  mountains,"  which  project 
above  the  broad  level  desert  surface,  and  which  are  in  reality 
the  summits  of  precipitous  mountains  that  have  been  almost 
completely  buried  beneath  these  recent  accumulations.  The 
prevailing  color  of  adobe  is  light  buff  to  gray,  excepting  when 
contaminated  with  organic  matter.  In  its  typical  form  it  is  so 
fine  as  to  be  quite  without  grit  when  rubbed  between  the  fingers. 
When  examined  under  the  microscope,  it  is  seen  to  be  com- 
posed of  irregular  unassorted  flakes  and  grains,  principally 
quartz,  but  fragments  of  other  minerals  are  also  present.  The 
adobe  of  Salt  Lake  shows  flocculent  masses  of  amorphous  matter, 
which,  when  thoroughly  disintegrated,  are  found  to  consist  of 
minute  sharply  angular  fragments  of  quartz  and  feldspar  with 
much  calcareous  matter,  and  only  rarely  a  shred  of  micaceous 
or  hornblendic  material.  In  size  the  particles  vary  from  those 
too  small  for  measurement  up  to  .08  millimetre  in  diameter. 

The  valuable  characteristics  of  the  adobe  are  its  extreme 
fineness,  great  depth,  and  wonderful  fertility. 

Although  comprising  the  soil  of  almost  the  entire  region  that 
was  but  recently  known  as  the  Great  American  Desert,  it  needs 
but  water  to  make  it  laugh  with  harvests.  While  its  physical 
properties  undoubtedly  have  much  to  do  with  its  fertility,  this 
quality  must  also  be  in  part  due  to  the  fresh  arid  undecomposed 
condition  of  its  constituent  parts.  Originating  doubtless  by 
purely  mechanical  agencies,  it  has  been  swept  by  winds  and 
spasmodic  rains  into  closely  adjacent  basins  occupied  by  but 
temporary  lakes,  where,  spread  out  over  a  floor  sometimes  almost 
absolutely  level,  it  has  been  subjected  to  a  minimum  amount  of 
leaching  and  retains  until  to-day  its  youthful  strength  and 
powers  of  recuperation.1  The  analyses  given  on  p.  334  will 
serve  to  show  the  varying  character  of  the  deposits  included 
under  this  name.  Especial  attention  need  only  be  called  to 
the  relatively  high  percentages  of  lime  and  the  alkalies. 

Under  the  head  of  alluvial  deposits  we  may  also  consider 
i  See  further  on  p.  369. 


334 


THE   KEGOLITH 


those  clay  accumulations  which  result  from  the  deposition  of 
fine  aluminous  sediments  sorted  by  running  streams  from  gla- 
cial debris  and  like  the  loess  laid  down  in  quiet  water,  though 
usually  estuarian  rather  than  lacustrine.  These  are  the  well- 
known  Leda  clays1  of  glacial  regions,  and  which  on  genetic 
grounds  might  well  be  classed  as  aqueo-glacial  deposits. 


CONSTITUENTS 

I 

II 

Silica  (Si02)  
Alumina  (A^Os) 

66.69  % 
14  16 

44.64  % 
13  19 

Ferric  oxide  (Fe20s)  

4.38 

5.12 

Manganese  oxide  (MnO)      

0.09 

0  13 

Lime  (CaO)    

2.49 

13.91 

Magnesia  (MgO) 

1  28 

2  96 

Potash  (K20) 

1  21 

1  71 

Soda  (Na20)  

0.57 

0  59 

Carbonic  acid  (C02)    

0  77 

8  55 

Phosphoric  acid  (P205)   

0.29 

0  94 

Sulphuric  anhydride  (S03)  
Chlorine  (Cl) 

0.41  . 
0  34 

0.64 
0  14 

Water  (H20)      

4  94 

3  84 

Organic  matter  

2.00 

3  43 

99.72% 

99.84  % 

I.  Adobe  from  Santa  F§,  New  Mexico.  II.  Adobe  from  Fort  Wingate,  New 
Mexico. 

Such  are  very  abundant  along  all  the  lower  valleys  of  the 
principal  rivers  of  New  England,  sometimes  coming  to  the  im- 
mediate surface  or  overlaid  with  a  thin  layer  of  sandy  material 
which,  together  with  a  little  organic  matter,  forms  the  true  soil. 
They  form,  according  to  Dawson,2  the  sub-soils  over  a  large  part 
of  the  great  plains  of  Lower  Canada,  varying  in  thickness  up 
to  50  or  even  100  feet,  usually  resting  upon  the  boulder  clay. 
They  are,  as  a  rule,  of  almost  impalpable  fineness,  unctious,  and 
extremely  plastic.  Excepting  where  superficially  oxidized  to 
buff  or  brown,  they  are  of  a  blue-gray  color  and  may  show  on 
analysis  considerable  quantities  of  lime  carbonate  and  alkalies, 
features  whereby  they  are  readily  distinguished  from  the  resid- 
ual clays,  and  which  are  regarded  as  indicative  of  an  origin  by 
mechanical  rather  than  chemical  means.  When  dried,  they  be- 
come greatly  indurated,  and  when  unmixed  with  other  mate- 
rials, bake  so  hard  during  seasons  of  drought,  or  are  so  plastic 
1  So  called  from  their  most  characteristic  fossil,  Leda.  2  The  Canadian  Ice  Age. 


THE   CHAMPLAIN   CLAYS 


335 


during  seasons  of  rainfall,  as  to  be  quite  uiisuited  for  cultivation. 
Mixed  with  varing  proportions  of  siliceous  sand  to  counteract 
shrinkage,  they  form  the  common  brick-making  materials  of  the 
Northeastern  states,  burning  red  and  brown. 

The  materials  of  the  Leda  clays  naturally  vary  in  different 
localities,  being  dependent  on  the  characteristics  of  the  rocks 
from  which  they  were  de- 
rived. Those  of  Canada, 
according  to  Dawson, 
were  derived  from  the 
waste  of  the  Utica  and 
Quebec  groups.  This 
authority  believes  that 
when  the  clay  was  in  sus- 
pension, it  was  probably 
of  a  reddish  or  brown 
color  from  the  iron  per- 
oxide it  contained,  but 
that,  like  the  bottom  mud 
now  forming  in  the  deeper 
parts  of  the  St.  Lawrence, 

the  coloring  matter  be-  FIG.  34.  -Showing  7article7  from  Leda  clays. 
Came  deoxidized  by  or-  1,  quartz ;  2,  orthoclase ;  3,  plagioclase ;  4,  mica ; 
ganic  matter  SO  soon  as  Mourmaline ;  6, pyroxene;  7, chlorite;  8, Horn- 

deposited,  the  iron  being 

converted  into  a  sulphide  or  protoxide  carbonate.  Inasmuch, 
however,  as  the  materials  were  so  largely  derived  by  the  grind- 
ing action  of  the  glaciers  on  fresh  rocks,  it  is  not  impossible 
that  they  may  have  been  again  deposited  as  clay  without  hav- 
ing ever  undergone  the  oxidizing  process. 

Unlike  the  till  or  boulder  clays,  these  Leda  clays  are  dis- 
tinctly stratified,  as  shown  in  the  accompanying  illustration. 
(PL  24.)  An  analysis  of  a  sample  from  this  locality  yielded 
the  author  results  as  given  in  column  I  on  p.  336.  In  column  II 
is  given  that  of  the  portion  (33.26  %)  soluble  in  hydrochloric 
acid  and  sodium  carbonate  solutions,  while  in  column  III  is  given 
the  composition  of  a  "semi-assorted  glacio-lacustrine "  clay 
bordering  on  Lake  Michigan  near  Milwaukee,  Wisconsin,  and 
in  IV  a  glacial  pebbly  clay  underlying  II  at  the  same  locality.1 


1  Analyses  II  and  III  from  Chamberlain  and  Salisbury's  paper,  6th  Ann.  Rep. 
U.  S.  Geol.  Survey,  1884-85. 


336 


THE   REGOLITH 
ANALYSES  OF  STRATIFIED  CLAYS 


CONSTITUENTS 

I 

II 

III 

IV 

Silica  (Si02)                           .... 

56.17  °L 

10.98% 

40.22  % 

48.81  % 

Alumina/  (A1203)           

24.25 

8.66 

8.47 

7.54 

Phosphoric  acid  (P205)    
Titanic  oxide  (Ti02)    
Ferric  iron  (Fe2O3) 

Not  det. 
Not  det. 
Not  det. 

Not  det. 
Not  det. 
Not  det. 

0.05 
0.35 

2.83 

0.13 
0.45 
2.53 

Ferrous  iron  (FeO) 

3.54 

5.191 

'  0.48 

0.65 

Manganese  oxide  (MnO)  
Lime  (CaO)     

Not  det. 
2.09 

Not  det. 
1.02 

Trace 
15.65 

0.03 
11.83 

Magnesia  (JVI0'0) 

2  57 

2.19 

7  80 

7.05 

Potash  (K20) 

4.06 

1  12 

2.36 

2.60 

Soda  (Na20)    
Water  (H2O)  

2.25 
4.69 

0.75 
3.65 

0.84 
1.952 

0.92 
2.  02  2 

Carbonic  acid  (C02)     
Organic  carbon  (C) 

None 
None 

None 
None 

18.76 
0  32 

15.47 
0.38 

Sulphuric  anhydride  (S03)  .... 

None 

None 

0.13 

0.05 

99.56% 

33.26  % 

100.21  % 

100.46% 

Related  to  the  delta  deposits  already  described,  but  differing 
in  that  their  inorganic  materials  are  in  large  part  derived  im- 
mediately from  the  sea,  are  due  to  the  transporting  and  assort- 
ing poAver  of  tide  and  wave  action,  are  the  salt-water  marsh,  or 
paludal  deposits  so  common  along  the  Atlantic  border  of  North 
America.  In  discussing  the  formation  of  these  and  their  grad- 
ual transitions  into  arable  lands,  we  cannot  do  better  than 
follow  Professor  N.  S.  Shaler.3 

The  formation  of  a  sea-coast  swamp  is  due  mainly  to  wave 
action  and  plant  growth.  It  is  dependent  upon  the  configura- 
tion of  the  coast.  Wave  action  upon  an  irregular  coast  such 
as  that  of  New  England  nearly  always  results  in  a  breaking  or 
wearing  away  of  the  exposed  headlands  and  the  transportation  of 
the  debris  from  these  into  intervening  inlets,  and  thrown  upon, 
or  at  least  in  a  direction  toward,  the  beaches.  On  these  beaches, 
as  one  may  any  day  observe,  the  rock  fragments  are  ever  being 
ground  smaller  and  smaller,  and  must  in  time  be  reduced  to 
the  condition  of  the  finest  sand  and  mud.  Each  incoming 

1  All  iron  determined  as  FeO. 

2  Contains  H  of  organic  matter  dried  at  100°  C. 

3  Ann.  Rep.  Director  of  the  U.  S.  Geol.  Survey,  1884-85. 


SEA-COAST   SWAMP   DEPOSITS  337 

wave  hurls  more  or  less  of  this  fragmental  material  upon  the 
beach,  whence  a  considerable  portion  of  it  may  be  again  carried 
seaward  by  the  bottom  current  or  undertow  as  the  wave  re- 
cedes. One  who  has  stood  upon  a  high  rock  on  the  sea-shore 
and  watched  the  waves  come  tumbling  at  his  feet  and  then  go 
creeping  silently  oceanward  once  more  cannot  have  failed  to 
notice  the  continual  seething  sound  due  to  the  constant  drag 
of  the  rock  fragments  one  over  the  other  as  they  are  impelled 
inward  and  outward  by  the  alternating  currents.  A  consider- 
able part  of  this  mud  is  taken  out  to  sea  by  the  undertow,  or 
bottom  current,  which  always  sets  from  a  storm-beaten  beach 
along  the  bottom,  but  another  part  is  urged  by  the  movement 
of  the  water  caused  by  the  waves  and  of  the  tidal  flow  into  the 
fjords,  where  it  falls  to  the  bottom.  In  this  process  of  carriage 
the  mud  is  generally  conveyed  along  the  shores  and  is  most 
commonly  deposited  in  the  parts  of  the  inlets  near  the  shore 
line.  Wherever  there  is  a  bay  within  which  the  tidal  current 
is  deadened  and  where  the  waves  have  little  play,  this  sediment 
is  most  rapidly  laid  down.  If  the  process  of  deposition  begins 
on  a  pebbly  bottom,  it  is  at  first  aided  by  the  irregularities  be- 
tween the  stones  and  the  friction  of  the  water  among  the  sea- 
weeds, which  frequently  attach  themselves  to  the  stones.  As 
soon  as  a  sheet  of  mud  is  established,  it  commonly  becomes 
occupied  by  a  dense  growth  of  eel-grass.  This  plant,  by  its 
habit  of  growth,  greatly  favors  the  deposition  of  sediment.  The 
separate  stems  are  set  very  closely  together,  the  interspaces  not 
generally  exceeding  1  or  2  inches.  A  tidal  current  of  2  miles 
an  hour,  swift  enough  to  carry  much  sediment,  is  almost  en- 
tirely deadened  in  this  tangle  of  plants. 

At  half  tide  on  the  New  England  coast  these  eel-grass  fields 
are  generally  covered  with  water  to  the  depth  of  several  feet ; 
at  this  stage  the  tidal  currents  are  commonly  strongest.  The 
water  above  the  level  of  the  grass  has  its  usual  freedom  of 
motion  and  brings  much  sedimentary  matter  above  the  level  of 
the  foliage.  As  the  tide  falls,  a  part  of  this  waste  is  entangled 
and  held  until  it  gradually  sinks  to  the  bottom,  so  that  each 
run  of  the  tide  gives  a  certain  contribution  of  sedimentary 
matter,  which  goes  to  shallow  the  water.  This  process  is  easily 
observed  from  a  boat  floating  over  a  field  of  these  plants.  The 
deadening  of  the  current  when  the  lowered  tide  brings  the  tops 
of  the  plants  near  the  surface  is  very  noticeable.  The  mass  of 


338  THE   KEGOL1TH 

floating  matter  —  mud,  fronds  of  sea-weed  (often  with  shells  or 
small  pebbles  attached  to  their  bases),  dead  fish,  and  a  mass  of 
other  refuse,  is  seen  to  collect  in  the  mesh  of  foliage  and  sink 
to  the  bottom.  The  dead  stems  of  the  eel-grass  and  the  bodies 
of  many  small  crustaceans  and  mollusca  which  live  on  its  stalks 
or  on  the  bottom  contribute  to  the  deposit,  so  that  it  thickens 
with  considerable  rapidity. 

When  the  bed  formed  on  the  sea-bottom  by  the  action  of  the 
eel-grass  and  its  associated  plants  has  risen  to  the  point  where 
it  is  dry  at  low  water  of  the  ordinary  run  of  tides,  the  eel-grass 
can  no  longer  maintain  itself,  but  gives  place  to  other  groups 
of  sea-weeds  and  grasses. 

These  species  of  plants  find  their  place  first  near  the  shore 
line,  where  the  eel-grass  platform  is  naturally  the  highest.  At 
first  their  vegetation  is  quite  sparse,  owing  to  the  difficulty 
with  which  they  endure  the  depth  of  water  at  high  tide.  There 
is  often,  indeed,  a  considerable  difficulty  in  establishing  the 
growth  of  the  second  group  of  plants,  and  for  a  while  the  de- 
posit takes  the  shape  of  bare  mud-flats,  dependent  in  the  main 
for  their  accumulation  of  detrital  matter  on  the  growth  of 
certain  mollusca,  especially  of  the  genera  Mytilus  and  Modiola. 


Ide 


FIG.  35.  —  Cross-section  of  marine  marsh,    a,  original  surface  of  shore  line ;  6,  grassy 
marsh;  c,  mud-flats;  d,  eel-grass;  e,  mud  accumulated  in  eel-grass. 

When,  as  is  usually  the  case,  the  more  highly  organized  plants 
have  difficulty  in  establishing  themselves  over  the  broad  sur- 
face of  the  mud-flat,  they  win  their  way  to  it  in  the  following 
manner. 

From  the  vantage  ground  of  the  shore  line,  where  these  plants 
easily  find  the  conditions  of  submergence  which  suit  their  needs, 
the  plants  slowly  extend  the  front  of  their  bench  out  over  the 
mud-flats.  (See  Fig.  35.) 

This  process  of  growth  can  be  more  easily  studied  than  that 
of  the  earlier  or  eel-grass  stage  of  the  marshes,  for  it  is  visible 
along  miles  of  our  sea-shore.  The  higher  grasses  have  even 


SEA-COAST   SWAMP  DEPOSITS  339 

more  thick-set  stems  than  those  of  the  eel-grass  flats ;  they  en- 
tangle sediment  even  more  effectively.  At  first  their  stems  are 
covered  for  a  few  hours  at  each  ordinary  tide ;  they  gather 
waste  rapidly,  and  soon  lift  the  plain  whichsthey  are  construct- 
ing up  to  the  point  where  only  at  the  highest  tides  are  the  tops 
covered  by  water.  At  this  stage  the  growth  of  the  deposit  is 
practically  arrested,  there  being  no  means  of  increase  save  from 
the  decay  of  the  grasses  themselves. 

"  On  the  central  parts  of  the  New  England  shore,  as  about 
Boston,  the  mud-flat  occupies  at  most  2  or  3  feet  in  the  alti- 
tude above  mean  low  tide  and  the  annual  addition  to  its  mass 
in  a  year  is  very  small,"  perhaps  not  so  much  as  the  tenth  of  an 
inch  in  a  year.  "  On  the  other  hand,  in  the  Basin  of  Minas, 
one  of  the  principal  inlets  leading  from  the  Bay  of  Fundy,  the 
contribution  of  sediment  is  so  great  that  vast  areas  have  been 
easily  reclaimed  from  the  sea  by  building  a  rude  enclosure 
around  an  area  of  the  higher  parts  of  the  mud-flat,  so  that  the 
speed  of  the  sediment-laden  waters  is  checked  and  they  are 
made  to  lay  down  their  burdens.  In  a  few  years,  often  in  a 
few  months,  this  enclosed  area  is  raised  to  near  the  level  of 
high  tides.  It  is  then  only  necessary  to  erect  a  barrier  suffi- 
cient to  exclude  the  tide,  with  gates  for  the  rain  water,  in  order 
to  have  the  land  completely  reclaimed  from  the  sea.  In  this 
simple  way  there  has  been  an  area  of  many  thousand  acres  of 
excellent  arable  land  created  along  these  shores."  l 

The  lithological  and  chemical  character  of  deposits  of  this 
nature  have  been  but  little  studied,  and  we  are  here  able  to  give 
only  two  analyses,  as  below,  in  which,  however,  it  is  probable 
that  the  matter  tabulated  as  insoluble  silica  includes  as  well  all 
silicates  insoluble  in  acid. 

Column  I  of  the  table  is  mud  from  the  marshes  of  Newport 
River,  a  few  miles  above  Beaufort,  in  Carteret  County,  North 

1  As  the  total  reclaimable  area  between  New  York  and  Portland  (Maine) 
probably  exceeds  200,000  acres,  their  money  value  in  their  best  state  will 
amount  to  at  least  $40,000,000,000.  The  cost  of  reclaiming  these  areas  and  re- 
ducing them  to  cultivation  should  not  exceed  the  fifth  part  of  that  sum.  It  may 
be  noted  that  from  the  chemical  composition  of  these  soils,  they  are  practically 
inexhaustible,  and  that  from  their  position  they  are  often  well  placed  for  irriga- 
tion. South  of  the  New  England  shore  the  marsh  area  is  much  more  extensive 
than  in  that  region.  It  is  probable  that  the  improvable  marshes  of  the  Atlantic 
coast  amount  to  at  least  3,000,000  acres  and  they  may  exceed  double  this  amount, 
(Shaler,  p.  380.) 


340 


THE   KEGOLITH 


Carolina.  This  marsh,  formed  by  the  filling  up  of  the  old  river 
channel,  several  miles  wide,  is  continually  enlarging  at  the  ex- 
pense of  the  water  surface  ;  and  similar  formations,  to  the  extent 
of  hundreds  of  square  miles,  are  accumulating  in  very  many 
shallow  bays  and  sounds  and  rivers  near  the  sea. 


CONSTITUENTS 

I 

II 

Silica  insoluble 

54  42  % 

72  70  °/ 

Silica   soluble 

'•"•«"   /o 

1  92 

Oxide  of  iron  and  alumina            

16.45 

5  69 

Lime      .          .                    .              

1.18 

1.39 

Magnesia    

0.07 

0.05 

Potash   

1.18 

1.82 

Soda 

0  79 

0  35 

Phosphoric  acid  .                   

0.25 

0.13 

Sulphuric  acid    

1.46 

0.33 

Organic  matter 

10  35 

Water    . 

20  92  \ 

Oxide  of  manganese    . 

0  54  J 

3.65 

Sulphide  of  iron      

1.09 

0  11 

Common  salt 

1  63 

1  71 

99.98  % 

100.10% 

Column  II  is  the  sea  mud  or  slime  which  is  deposited  in  the 
shoal  waters  of  Beaufort  Harbor  and  along  the  sounds  and  estu- 
aries of  the  North  Carolina  coast.  It  is  a  fine,  dark-colored 
salt  mud,  formed  of  the  silt  brought  down  by  the  rivers,  mixed 
with  decaying  vegetable  matter  (mostly  sea-weed  and  marsh 
grass),  and  animal  remains,  —  of  fish,  molluscs,  and  all  sorts  of 
marine  organisms.1 

What  is  described  by  Whitney 2  as  a  typical  swamp  bog  or 
peat  soil,  from  a  rice  field  near  Georgetown,  South  Carolina, 
yielded  the  results  as  below,  in  columns  I,  II,  III,  and  IV,  the 
last  two  being  simply  recalculated  from  columns  I  and  II  on  an 
organic  and  water-free  basis.  These  are  the  so-called  sob-field 
soils,  in  themselves  poor,  but  responding  readily  to  fertilizers. 
When  exhausted  by  cultivation,  they  recuperate  quickly  through 
the  aid  of  silt  deposits  from  the  rivers,  brought  about  by  the 
continual  ebb  and  flow  of  the  tides. 

1  Geology  of  North  Carolina,  Vol.  I,  1875,  p.  214. 

2  Rice,  Its  Cultivation,  Production,  and  Distribution,  Rep.  No.  6,  Misc.  Series, 
U.  S.  Dept.  of  Agriculture,  1893. 


BEACH   SANDS 


341 


DIAMETER 

I 

II 

III 

IV 

PARTICLES 

CONVENTIONAL  NAMES 

SOIL 

SUB-SOIL 

SOIL 

SUB-SOIL 

mm. 

0-6  inches 

6-9  inches 

0-6  inches 

6-9  inches 

2-1 

Fine  gravel 

0.00% 

0  00  % 

0  00  °/ 

0  00  °/ 

1-.5 

Coarse  sand    

0.71 

0.08 

1.86 

0.14 

.5-.25 

Medium  sand  . 

2.70 

0.25 

5  18 

0  43 

.25-.! 

Fine  sand  

0.83 

0.13 

1.59 

0.23 

.1-.05 

Very  fine  sand    .... 

0.37 

0.15 

0.71 

0.26 

.05-.01 

Silt 

10.32 

13  97 

19  79 

24  30 

.01-.005 

Fine  silt      

5.32 

7.10 

10.20 

14.09 

005-  0001 

Clay 

31.90 

34  85 

61  17 

60  65 

Total  mineral  matter  .     . 

52.15% 

57.53% 

100.00  % 

100.00  % 

Organic  matter,  water  loss 

47.85 

42.47 

100.00  % 

100.00  % 

Loss  by  direct  ignition     . 

47.36 

39.65 

Beach  Sands. — Although  differing  radically  in  composition 
from  the  sea-coast  swamp  deposits  already  described,  we  must,  on 
account  of  their  intimate  geological  relationship,  include  here  a 
brief  description  of  those  fragmental  deposits  formed  by  wave 
action  along  beaches  and  in  many  instances  almost  absolutely 
free  from  organic  matter  of  any  kind.  Such  are  the  clean 
white  beach  sands,  the  delight  of  the  summer  visitor  at  the  sea- 
sides. These  are  found  here  and  there  in  isolated  stretches 
along  the  Atlantic  slopes,  particularly  where,  as  at  Old  Orchard, 
Maine,  they  receive  the  full  sweep  of  wave  and  tide  from  the 
open  sea.  In  many  instances  the  material  forming  these  beaches 
is  siliceous  sand,  from  glacial  deposits  which  the  ocean  has 
reasserted  according  to  its  own  liking.  In  other  cases  it  is 
sand  brought  down  by  rivers,  and  which  has  undergone  frac- 
tional separation  through  the  varying  strength  of  transporting 
agencies.  In  still  others  it  is  material  derived  immediately 
from  the  shore  rocks  through  the  weathering  action  of  atmos- 
pheres and  the  hammering  of  the  waves.  In  other  cases  yet, 
as  along  the  coasts  of  Florida,  the  source  is  problematical.  We 
can  only  say,  knowing  the  character  of  rocks  forming  the  main- 
land, that  they  could  not  have  here  originated,  but  must  have 
been  transported  and  probably  down  the  coast,  from  the  areas 
of  crystalline  rocks  to  the  northward.  It  is  sometimes,  though 


342  THE   REGOLITH 

not  always,  possible  to  gain  an  idea  of  the  probable  source  of 
these  sands  through  a  study  of  their  mineralogical  nature  and 
the  physical  condition  of  the  individual  particles. 

Sorby,  who  devoted  careful  attention  to  the  microscopic  ap- 
pearance of  granules  of  quartz  sand  belonging  to  various  geo- 
logical periods,  divided  them  into  five  types,  "which  though 
characteristically  distinct,  gradually  pass  into  one  another."1 
These  types  are  :  - 

1.  Normal,  angular,  fresh-formed  sand,  as  derived  almost 
directly  from  granitic  or  schistose  rocks. 

2.  Well-worn  sand  in  rounded  grains,  the  original  angles 
being  completely  lost,  and  the  surface  looking  like  fine  ground 
glass. 

3.  Sand  mechanically  broken  into  sharp  angular  chips,  show- 
ing a  glassy  fracture. 

4.  Sand  having  the  grains  chemically  corroded,  so  as  to  pro- 
duce a  peculiar  texture  of  the  surface,  differing  from  that  of 
worn  grains  or  crystals. 

5.  Sand  in  which  the  grains  have  a  perfect  crystalline  out- 
line, in  some  cases  undoubtedly  due  to  the  deposition  of  quartz 
over   rounded  or  angular  nuclei    of   ordinary  non-crystalline 
sand. 

The  material  of  most  beach  sands  is  largely  quartz,  though 
this  is  not  invariably  so.  Those  of  the  Bermudas  are,  as  a 
matter  of  necessity,  calcareous.  Those  of  isolated  deep-sea 
islands  like  the  Hawaiians,  are  derived  in  part  from  the  vol- 
canic rocks  of  the  islands,  and  in  some  instances  are  composed 
almost  wholly  of  minute  shells  of  the  size  of  a  pin's  head. 
These  last  from  their  faculty  of  emitting  a  crunching  sound 
when  disturbed,  are  known  as  "  sounding  "  or  "  singing  sands." 

The  beach  sand  at  Diamond  Head,  Oahu,  is  mainly  of  olivine 
and  magnetite  granules,  with  smaller  amounts  of  calcareous 
matter.  As  usual,  the  grains  in  samples  from  the  same  level 
are  of  fairly  uniform  dimension,  varying  from  0.5—1.0  millime- 
tre, the  larger  forms  being  often  fairly  well  rounded,  while  the 
smaller  may  still  show  crystal  outlines.  The  granules,  even 
in  the  same  sample,  however,  vary  greatly  in  the  amount  of 
rounding  they  have  undergone.  Like  the  quartz  granules  from 
the  Florida  beach,  these  show  conchoidal  drippings  due  to  the 
shock  of  impact  as  one  granule  strikes  against  another. 

1  Proc.  Geol.  Soc.  of  London,  Anniversary  Address,  Session,  1879-80,  p.  58. 


BEACH   SANDS 


343 


FIG.  36.  —  Quartz  granules  in  sand  from  beach, 
Santa  Rosa  island. 


The  beach  of  Santa  Rosa  island,  south  of  Pensacola,  Florida, 
is  composed  of  clear  white  quartz  sand  of  almost  ideal  purity. 
The  grains,  though  water-worn  and  with  the  lesser  angles 
rounded,  are  still  in  many 
cases  angular,  and  of  very 
uniform  size  (about  .5- 
1.0  millimetre),  as  shown 
in  Fig.  36..  These  gran- 
ules offer  a  very  beauti- 
ful illustration  of  Sorby's 
type  No.  2,  the  surface  of 
each  one,  through  abra- 
sion, being  reduced  to  the 
condition  of  ground  glass. 
Examination  with  a  high 
power  brings  out  minute 
fractures  and  conchoidal 
chippings,  at  once  sug- 
gestive of  the  prelimi- 
nary stages  of  manufact- 
ure of  the  quartz  spheres 
for  which  the  Japanese  are  so  noted.  It  is  as  though  each 
granule  had  been  held  in  the  hand  of  some  pigmy  aboriginal, 
and  its  surface  reduced  by  hammering  with  another  pebble, 
after  the  manner  known  among  archaeologists  as  "pecking." 

The  shape  assumed  by  a  rock  or  mineral  fragment  subjected 
to  wave  action  varies  somewhat  with  the  nature  of  the  material, 
schistose  rocks  and  easily  cleavable  minerals  naturally  giving 
rise  to  pebbles  or  granules  of  quite  unequal  dimensions  in  three 
directions.  The  schist  on  the  coast  of  Cape  Elizabeth,  Maine, 
for  instance,  gives  rise  to  pebbles  in  the  form  of  a  greatly  flat- 
tened oval,  while  the  more  homogeneous  quartz,  with  which  it 
is  associated,  yields  nearly  spherical  forms.  "  But  of  whatever 
character  the  material,  the  normal  shape  of  a  beach-formed 
boulder  or  pebble  is  oval,  and  this  for  the  reason  that  the  wave 
action  is  a  dragging  rather  than  a  carrying  one  ;  the  stone  is 
not  lifted  bodily  and  hurled  toward  the  shore  to  roll  back  with 
the  receding  wave,  but  is  rather  shoved  and  dragged  along. 
Gravity  tends  to  hold  the  fragments  in  one  position  so  that 
the  wear  is  greatest  on  the  side  which  is  down,  and  this  in 
itself  would  cause  them  to  assume  an  oval  or  flattened  form 


344  THE   EEGOLITH 

even  were  they  spherical  and  of  homogeneous  material  at  the 
start."1 

(3)  JEolian  Deposits.  —  That  no  sharp  lines  can  in  all  cases  be 
drawn  between  alluvial  and  ceolian  deposits  has  been  made  evi- 
dent in  our  discussion  of  the  loess  and  adobe.  We  will  now  con- 
sider those  deposits  which  owe  their  origin  and  present  structural 
features  almost  altogether,  if  not  entirely,  to  wind  action. 

The  efficacy  of  the  wind  as  an  agent  of  transportation  was 
dwelt  upon  in  considerable  detail  on  pp.  184  and  292.  The 
material  thus  carried  into  the  air,  often  to  great  heights,  is 
brought  to  the  surface  again  by  gravity,  though  the  normal  rate 
of  descent  is  not  infrequently  greatly  accelerated  by  rain  or 
snow.  Indeed,  the  clearness,  limpidity,  of  the  atmosphere  after 
a  rainfall  is  due  simply  to  the  fact  that  it  has  been  washed,  is 
cleansed  of  its  suspended  impurities. 

The  very  fogs  which  infest  our  cities,  particularly  those  of 
the  soft  coal  regions,  are  but  indices  of  the  dust  particles  in  the 
atmosphere,  each  globule  of  fog  being  condensed  about  a  nucleus 
of  floating  matter. 

The  amount  of  this  dust  brought  down  even  from  moderately 
clear  atmospheres  is  often  sufficiently  abundant  to  attract  the 
attention  of  the  most  casual  observer.  Professor  H.  L.  Bruner 
of  Irvington,  Indiana,  has  stated 2  that  during  a  snowstorm  in 
February,  1895,  a  layer  of  snow  about  one-fourth  of  an  inch  in 
thickness  was  colored  distinctly  brown  by  the  dust  it  contained. 
One  sample  of  snow  collected  yielded  .37%  of  dust,  by  weight, 
and  it  was  calculated  that  dust  was  thus  deposited  at  the  rate  of 
30.7  pounds  avoirdupois  for  each  acre.  Another  observer  calcu- 
lated the  fall  as  taking  place  at  the  rate  of  12.77  pounds  per  acre. 

From  a  gallon  of  water  melted  from  a  snowfall  of  but  4 
inches,  which  fell  in  London  in  January,  1895,  there  was  obtained 
10.65  grains  of  solid  matter,  5.75  grains  being  inorganic  and 
4.90  grains  carbonaceous.  Water  from  a  snow  collected  near 
the  centre  of  the  city,  January  30  of  this  same  year,  gave  6.25 
grains  of  mineral  and  11.07  grains  of  carbonaceous  matter.  It 
was  also  found  that  75  %  of  these  impurities  were  brought 
down  with  the  first  2  inches  of  the  snowfall. 

Dr.    Whitney,   who   examined  samples    of   the   black   earth 

1  Merrill,  Preliminary  Handbook,  Dept.  of  Geology,  U.  S.  National  Museum, 
1889,  p.  23. 

2  Monthly  Weather  Review,  U.  S.  Dept.  of  Agriculture,  January,  1895. 


PLATE   24 


FIG.  1.   Section  of  beds  of  Leda  clay,  Lewiston,  Maine. 

FIG.  2.   Beds  of  volcanic  dust,  Reese  Creek,  Gallatin  County,  Montana. 


DEPOSITS 


345 


brought  down  near  Rockville,  Indiana,  during  a  snowfall  of  the 
winter  of  1895,  reported1  it  as  consisting  of  material  almost 
identical  with  the  prevailing  loess  of  that  region,  from  whence 
it  was  doubtless  derived.  The  individual  particles  varied  in  size 
between  .10  and  .05  millimetre.  The  results  of  a  mechanical 
analysis  of  the  dust  are  tabulated  with  those  of  loess  on  p.  331. 
Samples  of  the  same  dust  submitted  to  microscopic  examination 
were  found  to  consist  of  fully  96  %  silt  and  4  %  organic  matter, 
the  latter  consisting  mainly  of  fresh-water  algte,  diatoms,  fungi, 
cells  from  decayed  grasses,  and  shreds  of  woody  tissue. 

Hilgard,  who  has  examined  the  so-called  "  dust  soils "  of 
Oregon,  California,  and  Washington,  and  which  during  the  dry 
seasons  are  so  loose  and  fine  as  to  rise  in  clouds  at  the  merest 
puff  of  wind,  gives  the  following  tables  to  show  their 
chemical  and  physical  natures,  and  which  he  regards  as  fairly 
typical  for  soils  of  the  arid  regions  of  the  United  States.2 

CHEMICAL  ANALYSES  OF  DUST  SOILS 


I 

II 

III 

CONSTITUENTS 

ATATHNAM 
PRAIRIE, 
YAKIMA 
COUNTY, 
WASHINGTON 

RATTLESNAKE 
CREEK,  KITTI- 
TAS  COUNTY, 
WASHINGTON 

PLATEAU  ON 
WILLOW  CREEK, 
MORROW 
COUNTY, 
OREGON 

Insoluble  matter  

% 
71  67  ) 

% 
78  33  1 

% 
7921  1 

Soluble  silica   

sni76-78 

2  20  I80'53 

2  SO  I81'51 

Potash  (K2O)  . 

1  07 

0  70 

0  89 

Soda  (Na2O)    

035 

024 

005 

Lime  (CaO)     . 

200 

208 

1  37 

Magnesia  (MgO) 

1  34 

1  47 

1  08 

Brown  oxide  of  manganese  (Mn304)  . 
Peroxide  of  iron  (Fe203)  
Alumina  (A^Oa) 

0.04 
6.88 
791 

0.07 
6.13 
6  12 

0.06 
5.63 
602 

Phosphoric  acid  (P205)     
Sulphuric  acid  (S03)     

0.13 
0.02 

0.18 
0.02 

0.18 
0.03 

Water  and  organic  matter    .... 

2.82 

2.35 

2.55 

Total    .... 

99  33  % 

99  90  % 

99  35  % 

Humus    

4.10 

044 

Hygroscopic  moisture  .     . 

4.98 

320 

4.92 

1  Monthly  Weather  Review,  U.  S.  Dept.  of  Agriculture,  January,  1895. 

2  Bull.  No.  3,  Weather  Bureau,  U.  S.  Dept.  of  Agriculture,  1892. 


346  THE   REGOLITH 

MECHANICAL  ANALYSES  OP  DUST  SOILS 


CONVENTIONAL  NAME 

DIAMETER  OF 
PARTICLES 

I 

II 

III 

Clay 

0023     mm. 

0.93% 

3.59% 

1  27  % 

Fine  silt     
Silt    
Very  fine  sand 

.005-.  Oil 
.013-.  027 
027    05 

30.93 
3.20 

7  18 

13.06 
5.82 
27.37 

32.29 
12.75 
37  51 

Fine  sand. 

05-.  122 

21  88 

43.78 

10  92 

Medium  sand     

.122-.  5 

32.39 

4.57 

3.97 

96.57% 

98.18% 

98.72% 

Sand  Dunes.  —  The  influence  of  the  wind  in  the  formation 
of  sand  hills  or  dunes,  as  they  are  commonly  called,  has  received 
attention  on  p.  184.  A  few  words  more  regarding  their  physi- 
cal qualities  and  lithological  nature  are  here  essential. 

The  effect  of  the  single  whirlwind  or  it  may  be  that  of  the 
more  constant  air  current  for  days,  weeks,  or  even  months, 
may  be  from  a  geological  standpoint  comparatively  insignifi- 
cant ;  but  they  are,  nevertheless,  interesting,  and  at  times 
important.  In  certain  regions  of  the  West,  and  notably  in 
parts  of  the  Colorado  desert,  as  described  by  W.  P.  Blake,  in 
1853,  all  the  fine  loose  sand  on  the  surface  of  the  ground  is 
blown  away,  leaving  every  pebble  and  boulder  standing  out 
in  strong  relief  from  the  hard  sun-baked  soil,  or  ledge  of 
bed-rock. 

Under  favorable  conditions  the  material  thus  blown  along 
may  gather  in  the  form  of  dunes,  which  themselves  travel 
slowly  across  the  country,  ever  changing  their  outlines  like 
drifts  of  snow.  A  few  miles  north  of  Winnemucca  Lake,  in 
western  Nevada,  is  a  belt  of  these  dunes  described  by  geologist 
Russell1  as  fully  75  feet  in  thickness  and  about  40  miles  in 
length  by  8  miles  in  breadth.  These,  under  the  restless 
goading  of  the  winds,  are  constantly  varying  in  shape,  and 
though  moving  in  mass  probably  but  a  few  feet  a  year  have 
already,  in  more  than  one  instance,  made  necessary  the  splicing 
of  telegraph  poles  to  prevent  the  burial  of  the  wires.  Another 
range  of  sand  dunes,  at  least  20  miles  in  length,  and  forming 

1  Geological  History  of  Lake  Lahontan,  Monograph  XI,  U.  S.  Geol.  Survey, 


JEOLIAN  DEPOSITS  347 

hills  200  to  300  feet  high,  occurs  on  the  eastern  end  of  Alkali 
Lake  in  the  same  state.  On  the  eastern  shore  of  Lake  Michi- 
gan are  also  dunes  of  sand  sometimes  200  feet  in  height,  and 
which  at  Grand  Haven  and  Sleeping  Bear  have  drifted  over 
the  adjacent  woodlands,  leaving  only  the  dead  tops  of  trees 
exposed.  Similar  dunes  occur  frequently  on  the  Atlantic 
coast,  as  at  Hatteras,  Long  Island,  and  Cape  Cod.  The  island 
of  Bermuda  is  made  up  almost  altogether  of  coral  and  shell 
fragments.  These  are  washed  by  the  waves  upon  the  beach, 
dried  by  the  winds,  and  blown  gradually  inland,  thus  forming 
hills  in  some  cases,  as  stated  by  Professor  Kice,1  not  less  than 
250  feet  in  height.  In  other  instances,  as  at  Elbow  Bay,  on 
the  south  shore  of  the  main  island,  the  sand,  like  a  huge 
glacier,  has  quite  filled  a  valley,  and  still  progressing  in  a 
mass  some  25  feet  in  thickness,  is  covering  houses,  gardens,  and 
even  woodlands,  leaving,  as  at  Lake  Michigan,  only  the  trunks 
of  dead  trees  standing  partially  exposed  in  the  midst  of  sandy 
plains. 

One  of  the  most  interesting  and  remarkable  of  the  many 
regions  for  the  observation  of  sand  dunes,  lies  between  Bor- 
deaux and  Bayonne  in  Gascony,  and  which  has  been  admirably 
described  by  Reclus.2  The  sea  here  throws  every  year  upon 
the  beach  along  a  line  100  miles  in  length  some  5,000,000 
cubic  yards  of  sand.  The  prevailing  westerly  winds,  contin- 
ually picking  up  the  surface  particles  from  the  seaward  side, 
whirl  them  over  to  the  inland  or  leeward  slope,  where  they 
are  again  deposited,  and  the  entire  ridge  by  this  means  alone 
moves  gradually  inland.  In  the  course  of  years  there  have 
thus  been  formed  a  complex  series  of  dunes  all  approximately 
parallel  with  the  coast  and  with  one  another,  and  of  all  alti- 
tudes up  to  250  feet.  These  are  still  marching  steadily  inward, 
though  at  the  rate  of  but  3  to  6  feet  annually,  and  whole  vil- 
lages have  more  than  once  been  torn  down  to  prevent  burial, 
and  rebuilt  at  a  distance,  to  be  again  removed  within  200 
years.3 

The   lithological   nature    of   the   dunes  is   widely   variable, 

1  Geology  of  Bermuda,  Bull.  25,  U.  S.  National  Museum. 

2  The  Earth,  Atmosphere,  and  Life. 

3  The  church  of  Lege,  owing  to  the  encroachment  of  the  sand  dunes,  was  torn 
down  in  1690,  and  rebuilt  at  a  distance  of  "2\  miles  from  its  first  site.     By  1850 
the  dunes  had  traversed  the  intervening  space,  and  again  necessitated  its  removal. 


348  THE    KEGOLITH 

though  naturally  siliceous  sand  is  the  prevailing  constituent 
in  the  majority  of  cases.  J.  W.  Retgers  describes l  the  dune 
sands  of  Holland  as  consisting  principally  of  quartz  granules, 
together  with  those  of  garnets,  augite,  hornblende,  tourmaline, 
epidote,  staurolite,  rutile,  zircon,  magnetite,  ilmenite,  ortho- 
clase,  calcite,  and  apatite ;  and,  more  rarely,  microcline,  cor- 
dierite,  titanite,  sillimanite,  olivine,  kyanite,  corundum,  and 
spinel.  The  majority  of  these  minerals  occur  in  the  form  of 
well-rounded  granules,  though  many  of  the  garnets,  zircons, 
and  magnetites  show  quite  well-preserved  crystal  outlines.  It 
is  noticeable  that  these  sands  contain  no  mica,  although  the 
mineral  occurs  in  the  sea-sand,  from  whence  the  dunes  are 
derived.  Retgers  accounts  for  this  on  the  supposition  that 
during  the  transportation  of  the  material  the  mica  folia  be- 
come so  finely  shredded  as  to  be  sifted  out  from  the  heavier 
particles  of  sand,  and  quite  dissipated.  It  is  well  to  note  that 
the  abrasive  power  of  wind-blown  particles  is  greater  than 
among  those  carried  by  water,  since,  as  noted  by  Daubree,  a 
thin  intervening  film  of  water  may  serve  to  buoy  up  the  gran- 
ules, and  keep  them  apart.  To  this  fact  is  ascribed  the  angular 
nature  of  many  of  the  wind-blown  grains,  they  having  become 
shattered  through  the  shock  of  impact.  This  same  authority 
seems  to  think  that  with  wind-blown  sand,  as  with  water- worn 
material,  there  is  a  minimum  limit,  beyond  which  reduction 
in  size  of  particles  rarely  goes.  This  minimum  he  places  at 
about  .25  millimetre  in  diameter.  It  seems,  however,  more 
probable  that  attrition  may  go  on  to  an  almost  indefinite  limit, 
but  that  the  finer  and  lighter  materials  are  driven  farther 
away  —  perhaps  not  collecting  in  the  form  of  dunes  at  all  — 
leaving,  as  one  would  naturally  expect,  the  sands  of  any  one 
series  of  dunes  of  nearly  uniform  size. 

It  was  noted  by  Blake  during  the  surveys  of  the  railway 
routes  to  the  Pacific  that  the  wind-blown  sands  of  the  Colorado 
desert  were  sometimes  in  the  form  of  almost  perfect  spheres,  all 
their  sharp  edges  and  asperities  having  been  worn  away  by 
mutual  attrition.  The  grains  were  composed  mainly  of  quartz, 
agate,  garnet,  and  dark  granules  derived  from  the  debris  of  vol- 
canic rocks.  In  places  there  is  a  black  iron  sand,  and  usually 
a  considerable  proportion  of  lime  carbonate,  as  indicated  by  its 
brisk  effervescence  when  treated  with  acid.  The  sand  dunes  of 
1  Neues  Jahrbuch  fur  Mineralogie  u.  Geologie,  etc.,  1895,  1  B.  1st  Heft,  p.  22. 


AEOLIAN   DEPOSITS 


349 


the  Bermudas,  as  elsewhere  noted,  are  composed  wholly  of  cal- 
careous material  from  finely  comminuted  shells  and  corals,  while 
those  of  the  Sevier  desert  region  of  Utah,  as  described  by  Gilbert,1 
are  of  fine  gypseous  sand  formed  by  the  evaporation  of  the  water 
in  the  neighboring  play  a  lakes. 

Volcanic  Dust.  —  The  finely  comminuted  materials  ejected 
from  volcanoes  and  caught  up  by  atmospheric  currents,  as  de- 
scribed on  p.  153,  are  sometimes  carried  long  distances  to  be 
again  deposited  either  on  land  or  in  the  water,  forming  loose, 
often  flour-like  deposits  of  varying  thickness.  At  various  points 
in  Colorado,  Kansas,  Nebraska,  Montana,  and  other  of  the  West- 
ern states,  are  remnant  beds  of  fine  volcanic  dust  such  as  must 
originally  have  covered  many  square  miles  of  territory,  and  the 
materials  of  which  were 
derived  from  sources  now 
wholly  obscured.2  The 
illustration  given  on  PL 
24  is  from  a  photograph, 
taken  by  the  writer,  of 
one  of  these  beds  in 
the  lower  Gallatin  val- 
ley, Montana.  From  the 
height  of  the  man's  shoul- 
der to  his  feet  the  bed  is 
of  pure  glassy  dust,  very 
light  gray  in  color,  and 
so  fine  and  light  that 
when  thrown  into  the 
air  it  floats  away  at  the 
slightest  breath.  The  fig- 
ure given  shows  the  ap- 
pearance of  this  glass  as  seen  under  the  microscope.  Beds  of 
this  nature  upwards  of  4  feet  in  thickness  occur  underlying  the 
loess  or  surface  soil  along  the  Republican  River  in  Nebraska 
and  Kansas  and  even  as  far  east  as  Omaha  in  the  first-named 
state.  The  source  of  their  materials  is  problematical. 

Deposits  of  this  nature  thus  far  described  are  of  very  recent 
origin,  and  the  beds  loosely  coherent.     There  are,  however,  good 

1  Monograph  I,  U.  S.  Geol.  Survey,  1890. 

2  See  On  Deposits  of  Volcanic  Dust  and  Sand  in  Southwestern  Nebraska, 
Proc.  U.  S.  National  Museum,  Vol.  VIII,  1885,  p.  99. 


FIG.  37.  — Showing  outlines  of  shreds  of  volcanic 
dust,  as  seen  under  the  microscope. 


350 


THE   REGOLITH 


reasons  for  supposing  that  similar  processes  were  carried  on  in 
the  earlier  stages  of  the  earth's  history,  but  that  the  peculiar^ 
susceptible  deposits  have  since  undergone  such  extensive  altera- 
tion as  to  be  no  longer  recognizable  as  wind-drifted  materials. 
Where  the  material  still  exists  as  a  surface  deposit,  it  undergoes 
ready  decomposition  on  account  of  its  porosity  and  easy  perme- 
ability. The  character  of  the  resultant  soil  is  dependent  some- 
what upon  the  character  of  the  material,  which  varies  indefinitely. 
The  volcanic  dusts  are  as  a  rule  siliceous,  more  nearly  allied  to 
the  acid  potash  rocks  than  to  the  basalts. 

The  analyses  given  below  show  the  chemical  nature  of  (I)  a 
fine,  white,  almost  flour-like  pumice  dust  from  Harlan  County, 
Nebraska,  and  (II)  of  dune  sands  from  the  Pamlico  Peninsula, 
North  Carolina.  This  last  is  described1  as  a  tolerably  fine, 
nearly  white  sand  consisting  of  smooth,  well-rounded  grains, 
mainly  quartz,  but  containing  also  occasional  shell  fragments 
and  black  granules  of  iron  ore. 


CONSTITUENTS 

I 

II 

Silica  (SiOo) 

69  12  V 

92  12  V 

Alumina  (ALOs)      | 

1  7  AA    I 

K  oq 

Iron  oxide  (Fe203)  j 
Lime  (CaO)    

0.86 

1.13 

Magnesia  (MgO) 

0  24 

0  03 

Potash  (KoO)      . 

6  64 

0  64 

Soda  (Na20)  .... 

1  69 

0  35 

Sulphuric  acid  (SOg)  .     .     . 

0  33 

Ignition 

4  05 

0  60 

100.23% 

100.49% 

(4)  Glacial  Deposits.  —  Under  this  name  are  included  those 
drift  deposits  which  are  the  product  mainly  of  glacial  action, 
though  their  immediate  deposition  may  have  been  brought  about 
in  part  through  the  instrumentality  of  water.  The  strictly 
aqueo-glacial  materials  have  been  noted  under  the  head  of 
alluvial  deposits. 

Allusion  has  been  already  made  to  the  manner  in  which  gla- 
ciers erode  and  transport.  During  a  comparatively  recent 
period  in  geologic  history,  there  appears  to  have  come  over  a 

1  Geology  of  North  Carolina,  Vol.  I,  1875,  pp.  182-183. 


GLACIAL   DEPOSITS  351 

portion  of  North  America  a  gradual  loAvering  of  the  normal 
temperature  or  increase  in  the  annual  precipitation,  or  perhaps 
both,  until  the  condition  of  affairs  now  existing  in  northern 
Greenland  prevailed  as  far  south  as  the  39th  parallel  of  north 
latitude.  Now  whether  the  ice  sheet  extended  at  any  one 
time  over  the  area  outlined  below  or  whether  there  were 
periods  of  advancement  and  retreat ;  whether  the  glaciation 
was  produced  by  floating  ice  and  local  glaciers  as  argued  by 
certain  Canadian  geologists,  or  by  a  truly  continental  ice  sheet 
thousands  of  feet  in  thickness,  are  for  our  present  purposes  mat- 
ters of  slight  concern.  We  have  more  to  do  with  results  than 
methods.  Suffice  it  for  the  moment,  that  over  the  entire  north- 
eastern part  of  the  United  States  and  eastern  Canada,  all  the  ex- 
isting loose  materials  from  rock  decay  that  had  been  gathering 
for  untold  ages  was  carried  bodily  northward,  westward,  or  south- 
ward, as  the  case  might  be.  From  over  a  considerable  part  of 
southern  New  England  the  original  residual  soils  were  stripped 
and  dumped  into  the  Atlantic,  portions  of  the  transported  mate- 
rial still  protruding  above  sea-level  in  the  forms  known  now  by 
the  names  of  Nantucket,  No  Man's  Land,  and  Block  Island.  In 
process  of  this  transfer  the  rocks  were  planed  down  to  hard 
fresh  surfaces,  over  and  upon  which  were  deposited  new  mate- 
rials from  the  north.  It  follows  that  over  this  entire  glaciated 
area,  estimated  by  Upham  1  as  some  4,000,000  square  miles,  with 
the  exception  of  a  few  comparatively  insignificant  patches  here 
and  there,  scarcely  a  foot  of  clastic  matter  is  to  be  found  that 
is  truly  native.  Wherever  road  cuts  or  stream  erosion  favors, 
the  regolith  in  various  conditions  of  compactness  may  be  found 
lying  directly  upon  the  hard,  smooth,  and  striated  rock  with 
which  it  has  perhaps  no  affinity  in  composition  or  structure. 
The  rotten  and  mechanically  triturated  detritus  of  many  rocks 
from  many  sources  more  or  less  admixed  by  the  moving  glacier 
or  commingled  by  resultant  streams,  is  spread  out  to  form  the 
soils  on  lands  to  which  it  is  as  truly  foreign  as  are  the  emigrants 
who  land  to-day  upon  our  shores.  The  stone  wall,  built  of 
boulders  found  loose  in  the  field,  may  consist  of  granites,  dia- 
bases, schists,  or  shales  even  though  the  underlying  rock  may  be 
a  limestone  ;  or  the  wall  may  be  of  limestone  though  the  coun- 
try rock  be  a  gneiss,  or  slate.  A  similar  distinction  exists  in 
the  soil  itself,  which,  while  it  may  in  part  consist  of  the  material 
1  Ice  Age  in  North  America,  p.  579. 


352  THE   KEGOLITH 

of  these  boulders  in  a  finely  divided  state,  is  more  likely  to  con- 
sist of  detritus  of  softer  rocks  which  yielded  more  readily  to  the 
abrasive  force.  Sand  and  gravel  or  clay,  dust  or  mud,  black 
with  organic  matter  or  red-brown  from  iron  oxides,  the  ad- 
mixture is  ever  varying,  dependent  only  on  the  nature  of  the 
materials  to  the  north.  But  the  material  of  the  glacial  drift 
is  spread  out  over  the  land  in  a  manner  far  from  uniform  and 
under  conditions  widely  variable.  Following  Professor  Salis- 
bury1 and  others,  we  may,  according  to  its  physical  charac- 
ters and  method  of  deposition,  separate  the  deposits  into 
two  general  groups:  (l)*the  stratified  or  assorted  drift,2  and 
(2)  the  unstratified  or  unassorted,  the  first  having  been  laid 
down  under  the  influence  of  water  and  hence  showing  a  more 
or  less  stratified  condition,  while  the  second,  deposited  directly 
from  the  ice,  consists  of  a  heterogeneous  aggregate  of  coarse  and 
fine  materials  without  evident  marks  of  stratification.  The  two 
forms  are  not  always  readily  separable  nor  is  their  relative  posi- 
tion always  the  same,  either  one  not  infrequently  occurring  up- 
permost, and  "not  rarely  they  alternate  with  each  other  several 
times  between  the  surface  and  the  bottom  of  the  drift." 

A  large  part  of  the  drift  is  composed  of  this  unstratified  and 
unassorted  material,  consisting  of  clay,  sand,  gravel,  and  boulders 
in  ever-varying  proportions,  and  to  which  the  name  till  or 
boulder  clay  is  commonly  applied,  or  from  its  mode  of  deposi- 
tion, that  of  ground  moraine.  As  already  noted,  it  is  the 
material  carried  along  bodily  beneath  the  ice  sheet  and  left 
in  the  position  it  now  occupies  on  its  final  retreat.  This, 
entirely  unmodified  except  upon  the  immediate  surface  where 
it  has  become  converted  into  soil  through  the  agencies  else- 
where described,  forms  the  regolith  over  large  areas  of  the 
northeastern  portion-  of  America  and  of  northern  Europe  as 
well.  Where  as  yet  unaffected  by  oxidation,  it  is  of  a  gray 
or  blue-gray  color,  and  often  so  intensely  tough  and  hard  as  to 
necessitate,  in  process  of  excavation,  recourse  to  blasting.  The 
upper  portion,  through  percolation  of  meteoric  waters,  is  as  a 
rule  of  a  buff  or  brownish  color,  owing  to  oxidation  of  the 
ferruginous  constituents.  Through  the  combined  agencies  of 
this  oxidation,  of  plant  and  animal  life  and  of  cultivation, 
considerable  contrasts  in  both  physical  and  chemical  properties 

1  Ann.  Rep.  State  Geologists  of  New  Jersey,  1891. 

2  Here  included  in  large  part  with  the  aqueo-glacial  deposits. 


GLACIAL   DEPOSITS  353 

are  brought  about  between  the  superficial  and  deeper-lying 
portion,  which  are  commonly  recognized  by  the  terms  soil  and 
sub-soil  respectively  applied  to  them,  though  originally  they 
may  have  been  one  and  the  same  thing.  The  composition  of 
this  till  naturally  varies  with  the  character  of  the  rocks  from 
whence  it  was  derived.  It  may  have,  and  indeed  probably  has, 
in  most  cases.travelled  but  a  short  distance,  and  its  constituent 
particles  may  be  the  same  as  that  of  the  rocks  which  it  overlies, 
though  in  a  finely  divided  condition,  only  the  harder  and 
tougher  rocks  retaining  their  lithological  identity,  while  the 
more  friable,  like  the  shales  and  sandstones,  have  been  ground 
to  the  condition  of  clay  and  sand.  To  attempt  to  give  then 
the  composition  of  the  till  would  necessitate  its  study  and  analy- 
sis in  innumerable  localities,  —  an  endless  and  profitless  task. 
It  will  be  sufficient  to  here  describe  a  few  representative  occur- 
rences. In  nearly  all  till  the  boulders,  consisting  of  the  harder 
and  more  resistant  of  the  materials,  are  in  a  more  or  less  rounded 
or  rhomboidal  form,  with  their  surfaces  scarred  and  with  other 
marks  of  the  rough  treatment  to  which  they  have  been  subjected. 
They  are  in  fact  the  tools  with  which  the  glacier  has  done  its 
work,  and  these  scars  are  but  the  signs  of  wear.  Intermingled 
with  these  is  an  ever-variable  amount  of  finer  detritus  largely 
a  result  of  mechanical  abrasion.  Professor  W.  O.  Crosby,  who 
has  studied  in  great  detail  the  physical  properties  of  the  till 
about  Boston,  states  J  that,  excluding  the  larger  stones,  it  con- 
sists of  25  %  of  coarse  material  which  may  be  classed  as  gravel ; 
20  %  of  sand ;  40  to  45  %  of  extremely  fine  sand,  or  rock  flour, 
and  less  than  12  %  of  clay.  The  gravel  in  these  cases  consists 
mainly  of  pebbles  of  the  harder  and  more  massive  rocks  of  the 
region,  such  as  granite,  diorite,  diabase,  quartzite,  and  sandstone. 
In  passing  from  sand  to  gravel,  there  is  noted  an  increase  in  the 
proportional  amount  of  quartz,  in  clear  and  angular  or  sub- 
angular  forms,  due  mainly  to  the  disintegration  of  the  granite, 
quartzite,  and  sandstone  pebbles.  The  "  rock  flour  "  also  con- 
sists essentially  of  quartz.  The  most  striking  feature  here 
brought  out  is  the  very  small  proportion  of  actual  clay  material, 
which  varies  from  one-tenth  to  one-eighth  of  the  total  bulk. 

The  following  table,  as  given  by  F.  Leverett,  shows  the 
approximate  physical  condition  of  the  till  as  represented  by 
the  sub-soil  in  various  parts  of  Illinois. 

1  Proc.  Boston  Soc.  of  Natural  History,  1890,  p.  123. 
2  A 


354 


THE   REGOLITH 


55   25 


pq 


O 


(N     CO     O     CO     O 
CM      O      O      O 

i 

o 

00 
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GLACIAL   DEPOSITS  355 

The  till  is  not,  however,  always  spread  out  evenly  over  the 
land,  but  though  partaking  in  a  general  way  of  the  topography 
of  the  slopes  which  it  covered,  lies  much  deeper  in  certain 
places  than  others.  Indeed,  it  thickens  and  thins  out  very 
irregularly  and  in  many  places  fails  entirely  either  through 
having  never  been  deposited,  as  over  many  a  rocky  hillside  in 
New  England,  or  through  having  been  removed  by  running 
water.  Moreover,  there  are  found  in  certain  parts  of  the  drift- 
covered  areas  rounded  hills  of  very  symmetrical  form,  composed 
of  material  identical  with  the  till,  but  which  must  have  been 
deposited  under  slightly  different  conditions.  These  range  in 
height  up  to  200  or  300  feet,  though  rarely  more  than  half  that 
amount.  Such  forms  are  known  as  drumlins. 

The  moraines,  as  already  noted,  represent  those  portions  of 
the  ice  drift  which  gathered  near  the  edge  of  the  ice  sheet  in 
the  form  of  submarginal  accumulations,  to  be  left  as  broad  belts 
or  ridges  of  sand  and  gravel  on  its  retreat.  Such  with  refer- 
ence to  their  position  to  the  margin  of  the  ice  are  known  as 
terminal,  marginal,  and  frontal  moraines.  The  materials  of 
which  they  are  composed  represent  (1)  that  which  accumulated 
beneath  the  edge  of  the  ice  while  it  was  practically  stationary 
for  a  considerable  length  of  time ;  (2)  that  dumped  from  the 
surface  at  its  margin;  and  (3)  that  pushed  up  by  the  ice  sheet, 
in  front  of  itself  during  its  forward  movement.  Such  ridges 
are  not  sharp  as  a  rule,  but  broad  and  low,  it  may  be  from  a 
fraction  of  one  to  several  miles  in  width.  Unlike  the  subgla- 
cial  drift, — the  till,  —  the  materials  are  but  loosely  consolidated, 
and  but  a  small  part,  if  any,  of  the  boulders  show  the  scarred 
and  abraded  surfaces  so  characteristic  of  those  of  the  till  proper. 

This  frontal  moraine,  occupying  the  southern  and  western 
margin  of  the  glaciated  area,  forms  one  of  the  most  striking 
and  unique  of  geological  bodies.  Composed  of  materials  of  a 
most  heterogeneous  nature,  ever  varying,  and  limited  in  range 
of  variation  only  by  the  lithological  character  of  the  rocks  to 
the  northward  and  eastward ;  in  all  degrees  of  coarseness  and 
fineness,  from  boulders  of  many  tons'  weight  to  particles  too 
small  to  be  visible  to  the  unaided  eye,  only  obscurely  and  some- 
times scarcely  at  all  stratified  excepting  where  subsequently 
modified  by  running  water ;  in  the  form  of  broad  low  hillocks, 
domes,  and  ridges, — the  moraine  sweeps  in  an  interrupted,  sin- 
uous belt  from  eastern  Massachusetts  to  North  Dakota  and  over 


356  THE   KEGOLITH 

400  miles  into  British  America,  having  a  length,  in  all  its  wind- 
ings and  turnings,  of  not  less  than  3000  miles. 

The  water  arising  from  the  melting  ice  sheet  flowed  off,  in 
part,  over  the  surface,  forming  superglacial  streams,  or  in  part 
upon  the  surface  of  the  ground  beneath  as  subglacial  streams, 
of  which  last  the  river  Rhone  of  to-day  is  a  good  example. 
Presumably  also  a  portion  of  the  water  became  concentrated 
and  flowed  for  short  distances  in  the  mass  of  the  ice  itself, 
forming  thus  englacial  streams.  In  all  cases  the  running  water 
would  collect,  reassort,  and  variously  modify  the  rock  debris 
found  either  in  immediate  connection  with  the  ice  itself  or  at 
its  extremity,  in  the  terminal  moraines.  There  were  thus 
formed  hillocks  and  ridges  or  low  fan-shaped  masses  of  "  modi- 
fied drift."  The  sand,  gravel,  and  boulders  which  collected  in 
the  troughs  of  superglacial  streams  would,  on  the  final  melting 
of  the  ice,  be  deposited  as  ridges  running  essentially  parallel 
with  that  of  the  movement  of  the  ice  on  which  they  formed. 
Such  are  known  as  eskers,  or  osars.  Other  deposits  closely 
resembling  these  and  sometimes  confounded  with  them,  but 
formed,  it  is  believed,  only  by  swift  and  changeable  currents 
near  the  frontal  margin  of  the  ice,  present  often  a  rude  and 
disturbed  and  distorted  stratification,  and  are  known  as  kames. 
They  differ  from  the  eskers  in  their  outlines  as  well  as  positions 
with  reference  to  the  glacier  from 'whence  their  materials  were 
derived,  being  as  a  rule  in  the  form  of  hills,  rather  than  ridges, 
and  with  their  longer  axes  at  right  angles  with  that  of  the  ice 
motion. 

Beyond  the  margin  of  the  ice  and  its  terminal  moraines  are 
found  still  other  loosely  aggregated  deposits  of  a  similar  hetero- 
geneous nature  wliich  are  likewise  due  to  swiftly  running  water 
caused  by  the  melting  ice.  Such,  according  to  their  position 
and  form,  are  known  as  valley  drift,  morainic  or  frontal  aprons, 
and  overwash  plains. 

The  thickness  of  these  glacial  deposits  varies  greatly,  as  has 
been  already  indicated.  Variations  of  upwards  of  a  hundred 
feet  may  occur  within  the  limits  of  even  less  than  one  square 
mile.  Professor  Newberry  estimated  that  the  area  south  and 
west  of  the  Canadian  highlands  covered  with  glacial  drift  was 
not  less  than  1,000,000  square  miles,  and  that  its  average 
depth  would  not  be  less  than  30  feet.  Other  estimates  on 
deposits  in  Ohio,  Indiana,  and  Illinois  give  an  average  thickness 


PLATE   25 


i 


FIG.  1.  Section  of  glacial  till.  FIG.  2.  Glaciated  landscape. 


THE   SOIL  357 

in  these  states  of  62  feet.  In  extreme  cases  the  deposit 
has  been  found  to  extend  to  a  depth  of  300  to  500  feet.  Bell 
has  stated l  that  glaciation  of  the  surface  of  British  America  has 
been  almost  universal  in  the  regions  east  of  the  Rocky  Moun- 
tains, and  all  over  the  Palaeozoic  districts  west  and  south  of 
Hudson  and  James  Bay  the  average  depth  of  the  till  is  100  feet, 
and  perhaps  200  feet  in  Manitoba  and  the  northwest  territories. 
The  following  section  is  given  by  James  Geikie  2  as  showing 
the  varying  character  of  the  glacial  drift  and  its  interstratified 
interglacial  lacustrine  deposits:  — 

FEET    INCHES 

Sandy  clay 5  0 

Brown  clay  and  stones  (till) 17  0 

Mud 15  0 

Sandy  mud .31  0 

Sand  and  gravel 28  0 

Sandy  clay  and  gravel 17  0 

Sand 5  0 

Mud 6  0 

Sand 14  0 

Gravel 30  0 

Brown  sandy  clay  and  stones  (till)     ....  30  0 

Hard  red  gravel 4  6 

Light  mud  and  sand 1  8 

Light  clay  and  stones 6  6 

Light  clay  and  whin  block 26  0 

Fine  sandy  mud 36  0 

Brown  clay,  gravel,  and  stones 14  4 

Dark  clay  and  stones  (till)    ...  .68  0 

355        0 
3.    THE   SOIL 

There  remains  now  to  be  summarized  a  few  of  the  character- 
istics of  those  superficial  portions  of  the  regolith  to  which  the 
name  soil  is  commonly  applied,  and  these,  too,  only  in  direct 
relation  to  their  properties  as  soils,  since  as  integral  portions 
of  the  regolith  they  have  already  been  sufficiently  touched 
upon. 

(1)  The  Chemical  Nature  of  Soils. --The  prevailing  con- 
stituent of  any  soil,  whatever  its  source,  is  nearly  always  silica, 
with  varying  amounts  of  alumina,  oxides  of  iron,  lime,  magne- 
sia, and  the  alkalies.3  A  small  amount  of  organic  matter,  from 

1  Bull.  Geol.  Soc.  of  America,  Vol.  I,  1890,  p.  289. 

2  The  Great  Ice  Age,  3d  ed.,  1894,  p.  120. 

s  The  peat  deposits  furnish  almost  the  only  exception  to  this  rule. 


358  THE   REGOLITH 

extraneous  source,  is  usually  present.  This  prevalence  of 
silica,  as  may  be  readily  understood,  is  an  essential  conse- 
quence of  soil  formation  through  the  breaking  down  of  rocks 
by  the  processes  of  weathering,  whereby  all  but  the  most  in- 
destructible portions  are  lost. 

The  predominantly  inorganic  nature  of  any  soil  may  easily 
be  shown  by  fractional  separations,  made  either  by  washing, 
or  by  sieves  of  varying  degrees  of  fineness,  whereby  it  is 
brought  into  portions  of  like  size  and  weight  such  as  can  con- 
veniently be  submitted  to  microscopical  and  chemical  analyses. 
All  portions,  from  the  finest  dust  to  particles  of  such  size  as  to 
be  classed  as  pebbles,  will  thus  be  found  to  be  but  mineral 
matter;  particles  of  quartz,  feldspar,  shreds  of  mica,  and  other 
silicates  in  ever-varying  proportions  and  stages  of  alteration 
or  decomposition. 

Owing  to  the  destructive  nature  of  their  formation,  it  is  but 
natural  that  a  soil,  particularly  one  of  considerable  antiquity, 
should  but  slightly  resemble  the  parent  rock.  This  fact  was 
more  than  suggested  in  the  chapter  on  rock-weathering.  In 
order  that  its  significance  may  be  fully  comprehended,  the 
analyses  of  fresh  rock  and  corresponding  residual  material  from 
various  sources  are  given  in  the  table  on  p.  359. 

The  most  striking  of  the  dissimilarities  shown  by  this  table 
are,  as  is  to  be  expected,  those  of  the  limestone  soils,  as  in 
columns  I  and  II,  where  the  proportional  amounts  of  silica, 
iron,  and  alumina  are  increased,  roughly  speaking,  nearly  one 
hundred  fold,  while  the  amount  of  lime  carbonate  is  corre- 
spondingly diminished.  This  condition  of  affairs  is  still  further 
exaggerated  in  the  case  of  the  red  soil  of  Bermuda  (columns 
III  and  IV)  and  which  offers  particularly  favorable  opportuni- 
ties for  study,  owing  to  the  isolated  condition  of  the  islands 
and  the  consequent  freedom  from  danger  of  contamination  by 
other  than  local  drift. 

The  shells  and  corals  which  in  a  more  or  less  consolidated 
condition  form  the  entire  mass  of  these  islands,  although  es- 
sentially of  carbonate  of  lime,  are  nevertheless  not  entirely  so, 
carrying,  aside  from  the  magnesia,  about  1  %  of  inorganic  im- 
purities, chiefly  oxides  of  iron  and  alumina  and  earthy  phos- 
phates, which  are  practically  insoluble  in  the  water  of  rainfalls, 
with  which  alone  we  have  to  do  here.  As  time  goes  on,  the 
lime  is  slowly  leached  out  and  carried  away  into  the  ocean,  the 


CHEMICAL   NATURE   OF   SOILS 


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360  THE   REGOLITH 

insoluble  parts  remaining.  Throughout  the  centuries  of  decay, 
this  1  %  of  insoluble  impurities,  representing  but  one  ton  of 
earth  to  every  99  tons  removed,  slowly  accumulates  until  it 
forms  the  common  red  earth  of  the  islands.  Though  usually 
fertile,  in  numerous  instances  where  the  leaching  has  been  ex- 
cessive the  resultant  soil  is  so  rich  in  iron  and  other  deleterious 
constituents  as  to  be  quite  barren. 

There  are  few  more  impressive  facts  in  agricultural  geology, 
than  that  each  foot  in  depth  of  such  soil,  as  it  now  lies  at  our 
feet,  may  indicate  the  removal  of  at  least  100  feet  in  actual  thick- 
ness of  limestone.  In  other  words,  assuming  that  nothing  has 
been  lost  by  mechanical  erosion,  the  surface  of  the  ground  has 
been  lowered  this  much  in  bringing  about  the  present  conditions. 

From  what  has  gone  before,  it  is  obvious  that  soils  derived 
by  purely  mechanical  agencies  will,  if  unmixed  with  other  ma- 
terials, show  a  composition  closely  resembling  the  mother  rock, 
as  in  the  case  of  that  derived  from  granite  as  described  on  p.  207 
or  those  derived  from  argillites  and  siliceous  sandstones  ;  others 
in  which  chemical  agencies  prevailed  may  by  solution  and  other 
changes  have  so  far  lost  important  constituents  as  to  be  scarce 
recognizable  as  rock  derivatives  at  all.  Obviously  a  rock  mass 
containing  in  itself  none  of  the  elements  of  plant  food  cannot, 
merely  through  its  decay,  furnish  soil  of  appreciable  fertility. 
This  fact  is  well  illustrated  in  the  region  known  as  the  Bare 
Hills  north  of  Baltimore,  Maryland,  or  the  Chester  County 
Barrens  in  southern  Pennsylvania.  Both  regions  are  under- 
laid by  peridotites  —  rocks  rich  in  iron-magnesian  silicates,  but 
almost  wholly  lacking  in  lime,  potash,  or  other  desirable  con- 
stituents. Such  rocks  not  merely  decompose  very  slowly,  but 
the  stingy  product  of  such  decomposition  consists  only  of  hya- 
line forms  of  silica,  magnesian  carbonates,  or  silicates  and  fer- 
ruginous products  quite  devoid  of  nutrient  matter,  affording 
food  and  foothold  to  scanty  growths  of  grass  and  stunted 
shrubs.  That,  however,  a  rock  contains  all  the  desired  mate- 
rials, is  no  certain  indication  as  to  character  of  its  decomposition 
product,  since  in  this  process  of  decomposition  much  desirable 
matter  may  have  become  lost.  Nevertheless  most  soils  retain 
what  we  may  call  inherited  characteristics,  and  a  direct  com- 
parison whenever  possible  is  by  no  means  uninteresting,  as  will 
be  noted  later. 

It  need  scarcely  be  remarked  that  the  value  of  any  soil  de- 


CHEMICAL  NATURE   OF   SOILS  361 

pends  wholly  upon  its  capacity  for  plant  growth.  Hence  a 
satisfactory  treatise  on  the  subject  should  be  written  with  a 
view  to  showing  to  what  this  capacity  is  due,  and  what  are 
the  laws  governing  its  fertility  and  its  rejuvenation  when  that 
fertility  becomes  exhausted.  Such  a  method  of  treatment  is, 
however,  far  beyond  the  limits  of  the  present  work,  and  we  must 
content  ourselves  with  merely  touching  upon  a  few  of  the  most 
salient  points,  leaving  the  at  present  little  understood  subject 
of  fertility  for  other  and  abler  writers.  It  may  be  well  to  re- 
mark, however,  that  a  soil  left  to  itself  and  nature's  processes 
rarely  becomes  barren  or  exhausted  except  it  may  be  under 
changed  geological  conditions.  A  growing  organism  takes 
temporarily  from  the  soil  that  which  is  essential,  but  restores 
it  again  with  accrued  interest  in  the  form  of  carbonaceous  and 
nitrogeneous  matter  derived  from  the  atmosphere,  when  it  dies. 
Thus,  under  normal  conditions,  the  soil  grows  yearly  richer 
and  richer  and  capable  of  supporting  larger  and  more  luxuriant 
crops.  It  is  only  when  the  husbandman  comes  in,  and  by  his 
improvident  harvesting  robs  the  soil  not  merely  of  its  interest 
due,  but  of  a  part  of  the  principal  as  well,  that  bankruptcy 
results. 

For  a  long  period  the  fertility  of  a  soil  was  felt  to  be  dependent 
very  largely  upon  its  chemical  composition,  and  older  treatises 
and  reports  of  geological  surveys  are  filled  with  tables  of  analy- 
ses which  the  acquired  knowledge  of  years  now  shows  us  to  be 
almost  as  worthless  as  can  be,  either  for  the  purposes  for  which 
they  were  first  intended,  or  as  indicative  of  the  mineral  nature 
of  the  soil  itself.1  A  soil  which,  under  certain  conditions  of 
climate  or  moisture,  is  utterly  barren  may,  under  changed  con- 
ditions, be  fruitful  in  the  extreme,  as  has  been  repeatedly  de- 
monstrated in  the  case  of  the  so-called  American  deserts,  dreary 
stretches  of  aridity  given  over  to  sage  brush  and  a  few  degraded 
forms  of  animal  life,  but  which  need  only  moisture  to  cause 
them  to  laugh  with  harvests. 

1  The  common  practice  of  making  soil  analyses,  whereby  the  results  are  tabu- 
lated as  soluble  and  insoluble  (meaning  by  soluble  the  portion  extracted  by  boil- 
ing hydrochloric  acid)  and  putting  down  the  latter  as  silica  (or  sand)  and 
insoluble  silicates,  cannot  be  too  strongly  condemned.  It  means  nothing.  A 
growing  plant  is  capable  of  extracting  only  a  small,  and  as  yet  unknown,  portion 
of  that  taken  out  by  the  acid,  and  as  to  what  silica  and  insoluble  silicates  may 
be,  we  are  left  in  ignorance.  Such  analyses  can  be  of  use  to  neither  the  student 
of  soils  or  of  geology. 


362  THE   REGOLITH. 

Naturally,  a  soil  containing  in  itself  nothing  in  the  way  of 
available  plant  food  can  be  made  to  produce  crops  only  when 
the  needed  constituents  are  supplied.  Investigations  have, 
however,  shown  that,  though  varying  in  different  species,  the 
proportional  amount  of  food  demanded  by  plants  which  can  be 
supplied  by  the  atmosphere  and  meteoric  waters  is  very  large. 

It  seems  to  be  now  pretty  well  conceded  that  of  all  the  con- 
stituents found  in  soil  aside  from  moisture,  only  potash,  lime, 
magnesia,  phosphoric  and  sulphuric  acids,  can  be  considered 
absolutely  essential  as  plant  food.  The  ash  of  all  plants,  to  be 
sure,  contains  silica,  soda,  —  and  it  may  be  iron  and  other  min- 
eral ingredients, — but  such  are  to  be  regarded  as  accidental 
rather  than  otherwise.  Of  the  constituents  enumerated  as 
essential,  magnesia  and  sulphuric  acid  are  almost  invariably 
present  in  sufficient  quantities,  while  potash,  lime,  and  phos- 
phoric acid,  even  though  sufficiently  abundant  in  a  virgin  soil, 
are  liable  to  exhaustion  under  the  ordinary  methods  of  culti- 
vation. The  source  of  these  materials  has  been  shown  in  the 
previous  pages  and  need  here  be  only  touched  upon.  The 
potash  and  the  lime  must  have  come  originally  from  the  de- 
composition of  potash-lime-bearing  silicates,  as  the  feldspars  and 
micas,  amphiboles  and  pyroxenes.  The  original  source  of  the 
phosphoric  acid  was  undoubtedly  the  apatite  of  the  eruptive 
rocks,  though  now  to  be  found  in  bones  and  skeletons  of  ani- 
mals, whose  remains  become  entombed  in  sedimentary  rocks 
of  all  ages.  How  small  and  proportionally  insignificant  are 
the  percentages  of  these  constituents  in  any  soil,  fertile  or 
barren,  is  shown  in  the  table  below,1  in  which  are  given  the 
general  average  composition  of  a  large  number  of  soils,  seden- 
tary and  transported.  The  sulphuric  acid,  which  is  not  given 
in  this  table,  rarely  amounts  to  more  than  from  0.05  %  to  0.5  % 
when  calculated  as  sulphuric  anhydride  (SO3). 

So  small,  comparatively,  are  these  percentages,  that  it  is  rare, 
indeed,  to  find  a  soil  which  on  complete  analysis  will  not  be 
shown  to  contain  them  in  sufficient  proportion.  The  varying 
degrees  of  fertility  in  such  cases  are  due  then,  not  to  differ- 
ences in  ultimate  composition,  but  to  difference  in  combination 
of  these  elements  whereby  they  are  or  are  not  available  for 
plant  food,  aiid'to  physical  and  climatic  differences  as  well. 

iFrom  Part  A,  Vol.  II,  Part  II,  Chemical  Analyses,  Geological  Survey  of 
Kentucky,  p.  113. 


CHEMICAL   NATURE   OF   SOILS 


363 


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Potash  in  acid  extract  .... 

Potash  in  the  insoluble  silicates 

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364  THE   REGOLITH 

Naturally  a  growing  plant  can  take  up  only  that  which  is 
soluble  by  the  means  at  its  command.  A  high  percentage  of 
any  of  the  above  constituents  counts  for  little  when  they  are 
combined  in  the  form  of  difficultly  soluble  silicates.  A  granitic 
rock,  as  has  already  been  noted,  contains  locked  up  in  its  mass 
all  the  mineral  elements  necessary  for  a  fertile  soil,  but  remains 
barren  simply  because  these  are  in  a  condition  of  slight  solu- 
bility and  its  physical  structure  is  such  that  even  the  soluble 
portions  are  unavailable.  Pulverize  this  rock  sufficiently,  and 
it  will  become  immediately  available  for  soil,  though  naturally 
its  fertility  is  slight,  and  rendered  enduring  only  by  gradual 
decomposition.  It  is  of  course  possible,  that  by  nature's 
methods,  decomposition  and  incident  leaching  may  have  gone 
so  far  that  a  soil  on  the  immediate  surface,  though  derived 
from  rocks  rich  in  essential  constituents,  has  become  quite 
impoverished  and  barren.  This  is  especially  true  with  lime- 
stone residuals,  as  has  been  already  noted.  It  is  doubtless  to 
this  fact  that  is  due  the  enduring  qualities  of  the  glacial  till 
as  a  soil,  though  its  immediate  fertility  may  not  be  as  great 
as  one  of  sedentary  origin.  The  undecomposed  feldspathic 
and  other  mineral  particles  contained  by  the  till,  due  to  its 
mechanical  origin,  yield  up  slowly  but  continually  their  sup- 
ply of  plant  food,  and  such  a  soil  may  long  outlast  the  residual 
clays  of  non-glaciated  regions. 

The  soils  derived  from  deposits  of  modified  glacial  drift  are 
almost  invariably  sandy  or  gravelly  in  their  nature.  Such,  on 
account  of  their  easy  working  qualities,  great  porosity,  and 
ready  permeability,  are  commonly  known  as  light  soils,  even 
though  their  actual  specific  gravities  may  be  greater  than  the 
so-called  heavy  soils  of  the  ground  moraine.1 

1  Mechanical  analysis  of  a  glacial  soil  from  an  old  pasture,  Cape  Elizabeth, 
Maine,  yielded  results  as  below.  The  portion  selected  was  of  just  the  thickness 
turned  up  by  the  plough,  —about  7  inches.  In  color  it  was  dark  gray,  at  the 
immediate  surface  almost  black  from  organic  matter,  and  penetrated  throughout 
by  grass  roots.  Fine  angular  grains  of  white  quartz  were  the  most  conspicuous 
feature  on  macroscopic  examination.  Eight  hundred  and  thirty  grammes  of  this 
soil  on  sifting  yielded  :  (1)  2.5  grammes  gravel,  which  failed  to  pass  a  sieve  con- 
taining 8  meshes  to  the  lineal  inch.  This  consisted  mainly  of  angular  quartz  and 
cleavage  bits  of  feldspar  with  occasional  rounded  lumps  of  impure  limonite,  and 
not  completely  disintegrated  particles  of  granitic  rock.  (2)  40  grammes  coarse 
sand  retained  by  20-mesh  sieve  and  consisting  of  clear  glassy  and  white  opaque 
quartz  in  angular  and  sub-angular  fragments,  the  largest  forms  being  some  3 
millimetres  in  greatest  diameter  ;  cleavage  bits  of  white  and  pink  feldspar,  rarely 


CHEMICAL  NATURE   OF   SOILS  365 

There  is  many  an  humble  homestead  throughout  the  glaciated 
areas  of  North  America  whose  lack  of  worldly  prosperity  is  due 
to  the  dry  and  barren  soil  supplied  by  these  deposits  of  modi- 
fied drift.  On  the  other  hand,  there  are  numerous  regions,  like 
those  of  northern  Ohio,  where  a  light,  barren,  residual  soil  de- 
rived from  sandstone  has  become  enriched  by  an  admixture  of 
glacial  clays  from  the  north,  and  thus  brought  prosperity  to 
thousands  of  happy  homes.  Nature  works  out  her  own  com- 
pensations, impoverishing,  it  may  be,  here  but  correspondingly 
enriching  there. 

R.  H.  Loughbridge  has  shown 1  that  the  percentage  of  soluble 

folia  of  white  mica,  a  few  bits  of  mica  schist,  and  lastly  hard,  rounded  pellets  of 
indurated  silt  and  organic  matter.  (3)  170  grammes  retained  by  40-mesh  sieve 
and  consisting  of  a  clean  sand  composed  of  some  two-thirds  its  bulk  white  quartz 
particles  and  one-third  opaque,  partially  kaolinized  feldspathic  particles  ;  rarely 
any  mica  or  free  iron  oxides.  (4)  180  grammes  retained  by  60-mesh  sieve  and 
consisting,  like  the  last,  of  clean  quartz  and  feldspar  sand,  the  quartz  particles  in 
excess  of  the  feldspar,  and  rarely  a  little  mica.  (5)  82  grammes  retained  by  the 
80-rnesh  sieve.  This,  very  clean  sand  of  quartz  and  feldspar,  in  the  proportion 
of  about  |  quartz  and  f  feldspar.  (6)  150  grammes  retained  by  a  sieve  of  silk 
bolting  cloth  of  120  meshes  to  the  lineal  inch.  Like  the  last,  composed  almost 
wholly  of  bright  quartzes  and  somewhat  kaolinized  feldspars  with  scarcely  a 
trace  of  other  silicates.  (7)  185  grammes  which  passed  the  silk  bolting  cloth. 
This  was  submitted  to  washing,  the  lighter  finer  material  being  poured  off  as  silt. 
By  this  means  were  obtained  118  grammes  very  fine  sand  and  67  grammes  silt. 
The  fine  sand,  as  before,  showed  under  the  microscope  only  quartz  and  feldspars, 
the  quartzes  still  in  excess.  The  silt  to  the  naked  eye  consisted  of  a  light  brown, 
almost  impalpable  material,  which  the  microscope  revolved  into  quartz  and 
feldspar  particles  with  shreds  of  ferruginous  products  evidently  derived  from 
tlje  decomposition  of  iron-magnesian  silicates,  such  as  micas  or  amphiboles. 
(8)  Organic  matter,  19.5  grammes. 

A  bulk  analysis  of  the  air  dry-soil,  excluding  all  grass  and  roots,  yielded 
results  as  below  :  — 

Ignition  (water  and  organic  matter) 2.72% 

Silica 76.80 

Alumina  and  iron  oxides 14.04 

Lime 0.78 

Magnesia Traces 

Potash 2.87 

Soda  .     .       1.18 


98.39  % 

Such  a  soil  is  plainly  little  more  than  a  highly  quartzose  granite  or  gneiss  in  a 
pulverulent  condition  and  in  which  the  agencies  of  decomposition  have  scarcely 
begun  their  work.  Its  composition  could  have  been  almost  foretold  by  the 
microscopic  examination. 

*On  the  Distribution  of  Soil  Ingredients  among  the  Sediments  obtained  in 
Silt  Analysis,  Am.  Jour,  of  Science,  Vol.  VII,  1874,  p.  17. 


366 


THE   REGOLITH 


material  in  a  soil  rapidly  increases  with  the  degree  of  commi- 
nution; i.e.  the  finer  the  material  the  larger  the  proportional 
amount  of  soluble  matter,  and  hence  of  matter  available  as 
plant  food.  This  is  well  brought  out  in  the  following  table 
abridged  from  the  one  given  in  Mr.  Loughbridge's  original 
paper,  the  figures  in  the  upper  spacerof  each  column  indicating 
the  size  of  the  particles,  and  the  percentage  amount  of  each  as 
determined  by  fractional  separations. 


CONVENTIONAL  NAME  : 

CLAY 

FINEST  SILT 

FINE  SILT 

MEDIUM 
SILT 

COARSE 

SILT 

DIAMETER  OF  PARTICLES  : 

21.64% 

? 

23.56  % 
ram. 
.005-.011 

12.54% 
mm. 
.013-.016 

13.67% 
mm. 

.022^.027 

13.11% 
mm. 
.033-.03S 

CONSTITUENTS 

% 

% 

% 

% 

% 

Insoluble  residue  .... 
Soluble  silica  
Potash  (KgO)  .  . 

15.96 
33.10 
1.47 

73.17 
9.95 
0  53 

87.96 
4.27 
0  29 

94.13 
2.35 
0.12 

96.52 

Soda  (Na20)  .  .  . 

(1.70)1 

0  24 

0.28 

0.21 

Lime  (CaO)  

0.09 

0.13 

0.18 

0.09 

Magnesia  (MgO)   .... 
Manganese  (Mn02)    .     .  '  . 
Iron  sesquioxide  (Fe203)     . 
Alumina  (A1203)  .... 
Phosphoric  acid  (P20s)  . 
Sulphuric  acid  (SOs)  .     .     . 
Volatile  matter      .... 

1.33 
0.30 
18.76 
18.19 
0.18 
0.06 
9.00 

0.46 
0.00 
4.76 
4.32 
0.11 
0.02 
5.61 

0.26 
0.00 
2.34 
2.64 
0.03 
0.03 
1.72 

0.10 
0.00 
1.03 
1.21 
0.02 
0.03 
0.92 

.... 

Totals  
Total  soluble  constituents  . 

100.14 

75.18 

99.30 
20.52 

100.00 
10.32 

100.21 
5.16 

.... 

According  to  Hilgard,2  the  substance  which  assumes  com- 
manding importance  as  controlling  the  fertility  of  a  soil,  aside 
from  physical  conditions,  is  lime,  in  the  presence  of  which,  in 
adequate  proportions,  smaller  percentages  of  the  other  plant 
foods  will  suffice  for  high  and  lasting  productiveness,  than 
would  otherwise  be  the  case.  Since  lime  is  the  essential  con- 
stituent of  the  rock  limestone,  it  follows  that,  other  things 
being  equal,  a  "limestone  country  is  a  rich  country."  As  else- 
where noted,  however,  a  limestone  soil  may  have  become  so 

1  An  excess  of  original  amount,  due  to  the  addition  of  sodium  chloride  to 
produce  flocculation  of  clay  in  suspension. 

2  The  Relation  of  Soil  to  Climate,  Bull.  No.  3,  U.  S.  Weather  Bureau,  1892. 


CHEMICAL   NATURE   OF   SOILS  367 

leached  of  its  lime,  through  prolonged  decay,  as  to  be  benefited 
by  artificial  applications  of  this  same  constituent.  Lime  is, 
moreover,  so  generally  distributed  throughout  the  great  major- 
ity of  rocks  that  few  soils  would  be  lacking  in  this  constituent, 
were  even  a  small  proportion  of  the  original  amount  left  in  the 
residue  from  rock  decay,  instead  of  being  so  largely  removed 
in  solution. 

It  would  follow  from  this  that  the  composition  and  fertility 
of  a  soil  is  dependent  not  more  upon  the  character  of  the  rock 
mass  from  which  it  is  derived,  than  upon  the  prevalent  climatic 
conditions  under  which  it  originated,  the  general  average  tem- 
perature and  the  amount  and  distribution  of  the  rainfall  being 
particularly  important  factors.  This  branch  of  the  subject  has 
also  been  considered  in  some  detail  by  Hilgard,  to  whom  we  are 
indebted  for  the  only  satisfactory  resume.  Concerning  condi- 
tions of  temperature,  this  author  says  :  - 

"  Within  the  ordinary  limits  of  atmospheric  temperatures  all 
the  chemical  processes  active  in  soil  formation  are  intensified 
by  high  and  retarded  by  low  temperatures,  all  other  conditions 
being  equal.  This  being  true,  we  would  expect  that  the  soils 
of  tropical  regions  should,  broadly  speaking,  be  more  highly 
decomposed  than  those  of  the  temperate  and  frigid  zones. 
While  this  fact  has  not  been  actually  verified  by  the  direct 
comparative  chemical  examination  of  corresponding  soils  from 
the  several  regions,  yet  the  incomparable  luxuriance  of  the 
natural  as  well  as  the  artificial  vegetation  in  the  tropics,  and 
the  long  duration  of  productiveness,  offer  at  least  presumptive 
evidence  of  the  practical  correctness  of  this  deduction.  In 
other  words,  the  fallowing  action,  which  in  temperate  regions 
takes  place  with  comparative  slowness,  necessitating  the  early 
use  of  fertilizers  on  an  extensive  scale,  has  been  much  more 
rapid  and  effective  in  the  hot  climates  of  the  equatorial  belts, 
thus  rendering  available  so  large  a  proportion  of  the  soil's  in- 
trinsic stores  of  plant  food  that  the  need  of  artificial  fertilization 
is  there  restricted  to  those  soils  of  which  the  parent  rocks  were 
exceptionally  deficient  in  the  mineral  ingredients  of  special 
importance  to  plants  that  ordinarily  form  the  essential  material 
of  fertilizers."1 

1  While  the  action  of  frost  in  bringing  rock  masses  into  the  condition  of  soil 
is,  in  temperate  climates,  of  very  great  importance,  there  seems  to  be  a  limit 
beyond  which  it  accomplishes  little  in  the  way  of  directly  promoting  decomposi- 


§68  THE   REGOLITH 

Concerning  the  concentration  and  leaching  out  of  certain  con- 
stituents by  the  action  of  meteoric  waters,  the  same  authority 
says  :  — 

"  When,  however,  the  rainfall  is  either  in  total  quantity  or  in 
its  distribution  insufficient  to  effect  this  leaching,  the  sub- 
stances which  otherwise  would  have  passed  into  the  sea  are 
wholly  or  partially  retained  in  the  soil  stratum,  and  when  in 
sufficient  amount  may  become  apparent  on  the  surface  in  the 
form  of  efflorescences  of  4  alkali '  salts.  One  of  the  most  im- 
portant modifications  produced  by  scantiness  of  rainfall  on  soil 
formation  is  the  great  retardation  of  formation  of  clay  from 
feldspathic  rocks  (kaolinization)  and  the  sediments  derived 
therefrom.  As  a  result,  it  is  observed  that  the  soils  of  the 
Atlantic  slope  are  prevalently  loams,  containing  considerable 
clay,  and  even  in  the  case  of  alluvial  lands,  oftentimes  very 
heavy,  while  the  character  of  the  soils  of  arid  regions  is  pre- 
dominantly sandy  or  silty  with  but  a  small  proportion  of  clay, 
unless  derived  directly  or  indirectly  from  clay  or  clay  shales. 
In  the  former  case,  the  clay,  becoming  partially  diffused  in 
the  rain  water  when  a  somewhat  heavy  fall  occurs,  percolates 
through  the  soil  in  that  condition  and  tends  to  accumulate  in 
the  sub-soil,  the  result  being  that  almost  without  exception 
the  sub-soils  of  the  humid  regions  are  very  decidedly  more 
clayey  than  the  corresponding  surface  soils.  Not  only  does 
this  clay  water  tend  to  make  the  sub-soil  more  compact  and 
heavy,  making  it  less  pervious  to  water  and  air,  but  it  is  as- 
sisted materially  in  this  by  the  action  which  tends  to  leach  the 
lime  carbonate  out  of  the  surface  soil  into  the  sub-soil.  The 
accumulated  clay  is  thus  frequently  more  or  less  cemented  into 
a  'hardpan'  by  lime  partly  in  the  form  of  carbonate  and  partly 
in  that  of  zeolitic  (hydrous  silicate)  compounds,  adding  to  the 
compactness  of  the  sub-soil,  and  therefore  to  the  usual  specific 
difference  between  the  soil  and  sub-soil ;  viz.  the  deficiency  or 
absence  of  humus  and  the  difficulty  of  penetration  by  an  aera- 
tion of  the  roots  of  plants." 

For  these  reasons  the  soils  of  arid  regions,  even  though  con- 
taining the  same  materials,  are  often  of  uniform  physical  and 
chemical  character  to  great  depths.  The  soluble  salts,  as  car- 

tion,  and  presumably  disintegration  as  well.  Collier's  (8th  Ann.  Kep.  New 
York  Exp.  Station,  1889)  experiments  showed  that  47  successive  freezings  and 
taawings  of  a  soil  did  not  perceptibly  increase  the  percentage  of  soluble  potash. 


CHEMICAL   NATURE   OF   SOILS 


369 


bonate  of  lime  and  salts  of  potash  and  soda^  which  are  leached 
away  in  regions  of  great  average  humidity,  remain  in  those 
where  the  annual  precipitation  is  less,  or  where,  on  account  of 
its  uneven  distribution  throughout  the  warmer  months  of  the 
year,  its  permeability  and  consequent  leaching  action  is  less. 
Hilgard  brings  out  this  fact  prominently  in  tables  from  which 
that  below  is  condensed,  the  original  being  compiled  from  sev- 
eral hundred  analyses  of  soils  from  the  humid  regions  of  North 
and  South  Carolina,  Georgia,  Florida,  Alabama,  Mississippi, 
Arkansas,  Kentucky,  and  the  arid  regions  of  California,  Wash- 
ington, Montana,  Utah,  Colorado,  Wyoming,  and  New  Mexico. 


SHOWING  THE  PROPORTIONAL  AMOUNTS  OF  SOLUBLE  SALTS  IN  SOILS  OF  ARID 
AND  HUMID  REGIONS 


CONSTITUENTS 

ARID  EEGION 

HUMID  REGION 

Insoluble  residue 

69  681  % 

84  472  °/ 

Soluble  silica   ..... 

6  289 

3  873 

Potash    

0  825 

0  187 

Soda  

0  251 

0  071 

Lime 

1  645 

0  112 

Magnesia 

1  384 

0  209 

Brown  manganese  oxide 

0  056 

0  126 

Iron  peroxide 

5  431 

3  455 

Alumina      

7  309 

4.008 

Phosphoric  acid    
Sulphuric  acid      
Water  and  organic  matter 

0.144 
0.035 
5  585 

0.114 
0.065 
3  557 

Total                                          .     . 

99  978  % 

100  093  % 

Discussing  these  figures,  Professor  Hilgard  says  :  "  Concern- 
ing this  table  with  reference  to  the  lime,  a  glance  at  the  col- 
umns for  the  two  regions  shows  a  surprising  and  evidently 
intrinsic  and  material  difference  approximating  to  the  propor- 
tion of  1  to  14J.  This  difference  is  so  great  that  no  accidental 
errors  in  the  selection  of  analysis  of  the  soils  can  to  any  mate- 
rial degree  weaken  the  overwhelming  proof  of  the  correctness 
of  the  inference  drawn  upon  theoretical  grounds  ;  viz.  that  the 
soils  of  the  arid  regions  must  be  richer  in  lime  than  those  of 
the  humid  countries."  These  remarks  hold  good  also  for  the 
percentages  of  magnesia  and  the  alkalies.  From  the  fact  that 

2  B 


370  THE   REGOLITH 

in  humid  regions  the  more  soluble  constituents  are  leached 
out,  we  may  safely  infer  a  corresponding  proportional  increase 
in  the  insoluble  constituents.  This  is  also  made  manifest  by 
the  tables,  there  being  a  difference  of  nearly  15%  in  favor  of  the 
humid  regions.  The  table  shows,  further,  a  probably  greater 
proportion  of  zeolitic  material  in  the  soil  of  arid  regions,  the 
assumption  being  based  upon  the  percentages  of  soluble  silica. 
Concerning  this  difference,  the  author  says  :  — 

4 '  Nor  should  this  be  a  matter  of  surprise  when  we  consider 
the  agencies  which  are  brought  to  bear  upon  the  soils  of  the 
arid  regions  with  so  much  greater  intensity  than  can  be  the 
case  where  the  solutions  resulting  from  the  weathering  process 
are  continually  removed  as  fast  as  formed  by  the  continuous 
leaching  effect  of  atmospheric  waters.  In  the  soils  of  regions 
where  summer  rains  are  insignificant  or  wanting,  these  solu- 
tions not  only  remain,  but  are  concentrated  by  evaporation  to 
a  point  that  in  the  nature  of  the  case  can  never  be  reached  in 
humid  climates.  Prominent  among  these  soluble  ingredients 
are  the  silicates  and  carbonates  of  the  two  alkalies,  potash  and 
soda.  The  former,  when  filtered  through  a  soil  containing  the 
carbonates  of  lime  and  magnesia,  will  soon  be  transformed  into 
complex  silicates  in  which  potash  takes  the  precedence  of  soda, 
and  which,  existing  in  a  very  finely  divided  (at  the  outset  in  a 
gelatinous)  condition,  serve  as  an  ever-ready  reservoir  to  catch 
and  store  the  lingering  alkalies  as  they  are  set  free  from  the 
rocks,  whether  in  the  form  of  soluble  silicates  or  carbonates.1 
The  latter  have  still  another  important  effect.  In  the  concen- 
trated form,  at  least,  they  themselves  are  effective  in  decom- 
posing silicate  minerals  refractory  to  milder  agencies,  such  as 
calcic  carbonate  solutions,  and  thus  the  more  decomposed  state 
in  which  we  find  the  soil  minerals  of  the  arid  regions  is  intel- 
ligible on  that  ground  alone.  But  it  must  not  be  forgotten 
that  lime  carbonate,  though  less  effective  than  the  correspond- 
ing alkali  solutions,  nevertheless  is  known  to  produce,  by  long- 
continued  action,  chemical  effects  similar  to  those  that  are  more 
quickly  and  energetically  brought  about  by  the  action  of 
caustic  lime.  In  the  analysis  of  silicates  we  employ  caustic 
lime  for  the  setting  free  of  the  alkalies  and  the  formation  of 
easily  decomposable  silicates  by  igniting  the  mixture  ;  but  the 
carbonate  will  slowly  produce  a  similar  change,  both  in  the 
1  See  author's  remarks  on  page  374. 


CHEMICAL    NATURE   OF   SOILS  371 

laboratory  and  in  the  soils,  in  which  it  is  constantly  present. 
This  is  strikingly  seen  when  we  contrast  the  analyses  of  calca- 
reous clay  soils  of  the  humid  region  with  the  corresponding 
non-calcareous  ones  of  the  same.  In  the  former  the  propor- 
tions of  dissolved  silica  and  alumina  are  almost  invariably 
much  greater  than  in  the  latter,  so  far  as  such  comparisons  are 
practicable  without  assured  absolute  identity  of  materials." 

It  is  evident  from  the  above  that,  provided  the  amount  of  de- 
composition be  the  same,  the  soil  of  an  arid  region  may  contain 
a  larger  proportion  of  desirable  constituents  than  one  in  a  region 
of  considerable  annual  precipitation.  It  may,  also,  and  for  the 
same  reasons,  contain  a  larger  proportion  of  constituents  that 
are  positively  deleterious.  This  is  particularly  true  of  arid  and 
semi-arid  regions  of  poor  drainage,  like  the  Great  Basin  regions 
of  the  United  States,  where  salts  of  sodium  not  infrequently 
accumulate  to  such  an  extent  as  to  render  the  land  sterile  and 
barren  in  the  extreme. 

The  primary  origin  of  the  sodium  in  these  salts  lies  in  the 
soda-bearing  silicate  minerals  forming  the  rocks  of  the  region 
and  from  which  they  have  been  set  free  through  their  decom- 
position. 

It  should  be  stated,  however,  that  the  so-called  "  alkali "  is 
not  composed  wholly  of  sodium  compounds,  but  contains  also 
salts  of  magnesia,  lime,  iron,  and  potash.  Nor  is  the  form  under 
which  the  salts  exist  at  all  constant.  As  a  rule,  the  larger  por- 
tion of  the  alkali  is  in  the  form  of  sulphate  of  soda,  though  a 
considerable  portion  may  exist  as  carbonate  or  chloride,  and 
smaller  proportions  in  the  form  of  nitrates.  Concerning  the 
formation  of  these  carbonates,  Hilgard  says : 1  — 

"  There  seems  to  be  a  consensus  of  opinion  that  the  carbona- 
tion  of  the  soda  is  connected  in  some  way  with  the  presence 
of  limestone  or  carbonate  of  lime,  and  that  an  exchange  has 
occurred  in  which  either  common  salt  or  Glauber  salt  have 
transferred  their  acidic  components  to  lime  and  have  become 
carbonates  instead.  .  .  .  Yet  the  simple  explanation  of  the 
contrary  reaction  was  given  and  published  as  early  as  1826  by 
Schweigger.  In  1859  it  was  again  observed  by  Alex  Muller, 
in  a  different  form,  but  neither  of  these  chemists,  nor  any  of 
their  readers,  appear  to  have  perceived  the  important  bearing  of 
this  reaction,  not  only  upon  the  formation  of  the  natural  clepos- 
1  Bull.  No.  3,  Weather  Bureau,  U.  S.  Dept.  of  Agriculture,  1892. 


372  THE    REGOLITH 

its  of  carbonate  of  soda,  but  also  upon  a  multitude  of  processes 
in  chemical  geology.  Without  going  into  details  ...  it  may 
be  broadly  stated  that  the  formation  of  carbonated  alkalies  oc- 
curs whenever  the  neutral  alkaline  salts  (chlorides  or  sulphates) 
are  placed  in  presence  of  lime  or  magnesia  carbonates  and  car- 
bonic acid,  or  of  alkali  '  supercarbonates '  (hydrocarbonates) 
containing  even  a  slight  excess  of  carbonic  acid  above  the  nor- 
mal carbonates,  the  latter  being  the  actual  condition  of  all 
natural  sodas." 

We  have  thus  far  considered,  only  those  elements  of  the  soil 
that  are  derived  directly  from  the  rocks  from  which  they  are 
formed. 

To  this  list  we  should  add  the  element  nitrogen,  not  so  much 
on  account  of  its  quantity,  as  its  value  as  plant  food  and  of  the 
great  economic  value  of  some  of  its  compounds.  The  common 
forms  under  which  this  element  exists,  are  (1)  atmospheric 
nitrogen,  a  colorless,  tasteless,  and  innocuous  gas  and  which 
forms  some  three-fourth  by  bulk  of  the  air  we  breathe,  and 
(2)  the  nitrogen  of  the  soil,  where  it  exists  in  at  least  three 
distinct  forms,  (1)  organic  nitrogen,  (2)  as  ammonia  or  ammonia 
salts,  and  (3)  as  nitric  acid. 

The  average  amount  of  nitrogen  present  in  agriculture  soils 
is  given  by  authorities  as  varying  from  0.1%  to  0.3  %,  though 
occasionally,  as  in  certain  soils  rich  in  organic  matter,  4  or  5  % . 
Of  these  forms  only  the  ammonia  salts  and  nitric  acid  are  of 
direct  value  for  plant  food.  Nitrogen,  in  the  form  of  nitrate 
of  soda,  forms  an  important  mineral  fertilizer,  as  noted  on  p.  71. 

The  extraordinary  richness  in  nitrates  of  the  soils  in  tropical 
countries,  and  particularly  in  South  America,  has  often  been 
remarked  since  the  subject  was  first  broached  by  Humboldt 
and  Boussingault.  According  to  Miintz  and  Maracano,  nitrates 
occur  in  the  soils  of  Venezuela,  the  valley  of  the  Orinoco,  and 
other  localities  sometimes  to  the  amount  of  30  %  of  their  mass. 
These  nitrates  they  show  to  be  due  to  the  oxidation  of  organic 
nitrogen  through  the  agency  of  bacteria.  They  state  that  in 
the  caverns  of  the  regions,  a  guano  composed  mainly  of  the 
excreta  of  birds  and  bats,  but  admixed  also  with  the  dead  bodies 
of  these  and  other  animals,  has  accumulated  to  the  amount  of 
millions  of  cubic  metres.  Through  the  gradual  nitrification  of 
this  guano,  and  a  combination  of  the  nitrogen  with  the  lime 
of  bones,  or  existing  as  a  carbonate  in  the  soil,  a  gradual  tran- 


MINERAL  NATURE   OF   SOILS 


373 


sition  is  brought  about  wherever  there  is  free  access  of  air  or 
the  temperature  is  sufficiently  high  to  stimulate  the  nitrifying 
organisms  to  their  fullest  activity.  There  is  thus  a  gradual 
change  in  the  character  of  the  nitrogeneous  combination  from 
the  interior  to  the  exterior  portions  of  the  cave,  as  shown  in  the 
following  analyses :  — 


CONSTITUENTS 

GUANO  FROM 
INTERIOR  OF 
CAVE 

EARTH  FROM 
THE  ENTRANCE 

EARTH  FROM 
SOME  DISTANCE 
FROM  ENTRANCE 

Organic  nitrogen     

11-74% 

2.41  % 

0.80% 

Nitrate  of  lime  

0.00 

3.03 

10.36 

These  authorities  would  account  for  the  presence  of  extensive 
deposits  of  nitrates  as  in  Chili,  on  the  assumption  that  the  solu- 
ble nitrate,  originally  derived  from  decomposing  organic  matter, 
as  noted  above,  had  been  leached  out  from  its  place  of  origin 
by  percolating  water  and  redeposited  elsewhere  on  evaporation. 
The  invocation  of  atmospheric  electricity  to  account  for  any 
part  of  the  nitrates  of  the  soils,  they  regard  as  quite  unneces- 
sary, the  same  being  of  indirect  influence  only,  furnishing  first 
nitrogen  for  growing  plants  which  in  their  turn  serve  as  food 
for  animals.  These  same  authorities  give  the  following  figures 
relative  to  the  amounts  of  nitrates  and  nitrogen  in  South 
American  soils  :  — 


CONSTITUENTS 

SAN  JUAN 

Los  MORROS 
DE  PARAPARA 

EL  ENCANTADO 

Nitrate  of  lime    . 

2.85% 

3.50% 

0.62  % 

Organic  nitrogen     

0.15 

0.27 

0.21 

(2)  The  Mineral  Composition  of  Soils. — This  is  essentially 
the  same  as  that  of  the  regolith  of  which  the  soil  forms  a  part. 
Fragmental  quartzes  and  feldspars  form  the  larger  proportion 
of  most  soils.  These  are  intermingled  with  shreds  of  mica, 
amphibole,  pyroxene,  calcite  or  aragonite,  iron  and  manganese 
oxides,  and  in  variable  proportions,  kaolin  and  other  silicates, 
carbonates  and  oxides.  The  presence  of  these  constituents  is 


374  THE   REGOLITH 

usually  somewhat  obscured  by  iron  oxides  and  carbonaceous 
matter ;  but  when  these  are  removed  by  acids  or  by  ignition, 
and  the  residue  submitted  to  microscopic  analyses,  the  true 
mineral  nature  can  be,  as  a  rule,  made  out  with  approximate 
accuracy.1 

From  what  has  gone  before,  it  must  be  evident  that  the  con- 
stituents of  any  soil  are  almost  universally  in  a  finely  fragmen- 
tal  condition,  a  few  of  the  smaller  more  resisting  minerals,  like 
the  rutiles,  tourmalines,  zircons,  etc.,  having  perhaps  escaped 
the  comminuting  processes.  Of  the  silicate  minerals  we  may 
be  sure  that  many  are  in  an  advanced  stage  of  hydration  and 
the  ferruginous  constituents  in  a  state  of  peroxidation.  It  is 
possible  that  under  favorable  conditions  new  minerals  of  fairly 
perfect  crystalline  development  may  be  temporarily  formed. 
Since  the  work  of  Lemberg  was  made  public,2  it  has  been  very 
commonly  assumed  that  various  minerals  of  the  zeolite  group 
were  present  and  exercised  an  important  function  in  the  con- 
servation of  soil  fertility.  Notwithstanding  the  somewhat 
enthusiastic  endorsement  by  Hilgard,  of  this  idea,  as  set  forth 
in  the  previous  pages,  the  writer  can  but  feel  that  too  much  has 
been  assumed,  both  regarding  their  actual  presence  and  their 
possible  utility. 

We  must  not  lose  sight  of  the  fact  that  the  actual  occurrence 
of  zeolites  in  soils,  where  they  have  been  formed,  is  as  yet  not 
proven.  Their  presence  is  inferred  from  the  fact  that  weak 
acids,  such  as  are  known  to  be  capable  of  decomposing  zeolitic 
minerals,  will  extract  from  the  soil,  among  other  constituents, 
certain  ones  which  are  characteristic  of  minerals  of  the  zeolitic 
group;  and  it  is  assumed,  purely  for  lack  of  a  better  reason,  that 
these  elements  are  those  thus  combined.  Even  if  this  be  true, 
their  efficacy  as  potash  holders  may  well  be  questioned,  since 
potash  is  not  as  a  rule  an  element  of  great  importance  in  zeo- 
litic minerals.  Out  of  the  23  known  species  of  zeolites  (includ- 
ing apophyllite),  in  but  five  is  potash  considered  an  essential 
constituent.  These  five,  as  already  noted  on  p.  32,  are  apo- 
phyllite, ptilolite,  mordenite,  phillipsite,  and  harmotome,  of 
which  phillipsite  alone  carries  upwards  of  6  %  (theoretically), 

1  See  Anleitung  zur  Mineralogischen  Bodenanalyse,  etc.,  by  Franz  Steinriede, 
Inaug.  Dis.  Friedrichs-Universitat  Halle- Wittenberg.    Halle,  1889. 

2  Zur  Kenntniss  der  Bildung  und  Umbildung  von  Silicaten,  Zeitschrift  der 
Deutschen  Geolischen  Gesellschaft,  Vols.  XXXVII  and  XXXVIII,  1885  and  1887. 


MINERAL   NATURE   OF   SOILS  375 

the  other  smaller  amounts,  the  average  for  the  five  being  about 
4  %.  Now  assuming  that  all  the  zeolites  in  the  soils  belonged 
to  these  five  groups  and  none  to  the  18  potash-free  varieties, 
and  that  10  %  of  any  soil  consisted  of  zeolitic  material,  even 
then  we  have  thus  combined  only  0.4  %  of  K2O. 

We  must  remember,  further,  that  the  zeolites  are  invariably 
secondary  minerals,  as  already  noted,  and  as  such  are  com- 
monly regarded  as  decomposition  products.  This  does  not 
necessarily  mean,  however,  that  they  are  products  of  superficial 
weathering.  Indeed,  in  the  majority  of  cases  the  evidence  is 
all  to  the  contrary,  they  being  plainly  a  result  of  deep-seated 
processes  going  on  in  the  rock  masses  long  before  atmospheric 
action  began  to  manifest  itself.  (See  under  Hydrometamor- 
phism,  p.  161.)  It  is  even  questionable  if  the  conditions  preva- 
lent in  soil  are  not  unfavorable  rather  than  otherwise  to  the 
formation  of  zeolitic  compounds,  and  if  such  traces  as  there 
exist  are  not  rather  residuals  from  the  breaking  down  of  rock 
masses  in  which  they  had  been  previously  formed. 

In  this  connection  it  is  well  to  remember  that  zeolites  as  a 
whole  are  characteristic  of  basic  eruptive  rocks,  such  as  have 
yielded  but  a  proportionately  small  amount  of  our  soils.  Also 
that  the  mutual  chemical  reactions  that  may  go  on  in  a  rock 
mass  due  to  close  juxtaposition  of  the  various  minerals  may 
largely,  if  not  entirely,  cease  in  a  soil  where  the  amount  of  in- 
terspace is  so  enormously  exaggerated. 

The  researches  made  during  the  Challenger  Expedition1 
show,  it  is  true,  that  even  at  so  low  temperatures  as  from  2°  to 
3°  C.  phillipsite  is  being  formed  in  the  deep-sea  muds  of  the 
Central  Pacific  and  Indian  oceans.  But  in  these  cases  the 
mud  is  the  finely  comminuted  debris  from  basic  eruptive  rocks, 
itself  peculiarly  liable  to  decay,  and  containing  all  the  materials 
necessary  for  zeolitic  formation.  It  is,  moreover,  in  a  condition 
of  continual  moisture  and  under  the  weight  of  the  thousands 
of  fathoms  of  overlying  water  which  is  here  in  a  state  of  ex- 
treme quiescence,  being  beyond  the  influence  of  superficial  move- 
ments, as  waves,  tides,  and  currents.  These  conditions  are  so 
widely  different  from  those  which  exist  in  the  superficial  parts 
of  land  areas,' that  they  can  be  regarded  as  merely  suggestive. 
The  same  may  be  said  relative  to  the  zeolite  (phillipsite  and 
apophyllite)  formations  at  Plombieres  as  described  by  Dau- 
1  Rep.  on  the  Scientific  Results,  1873-76,  Deep-sea  Deposits,  1891,  pp.  400-411. 


376  THE   EEGOLITH 

bree.1  Another  fact  which  mitigates  against  the  theory  of 
zeolitic  formation  in  soils,  is  the  almost  universal  absence  of 
these  minerals  in  such  secondary,  unmetamorphosed  rocks  as 
are  the  product  of  the  reconsolidation  of  the  same  class  of  ma- 
terials as  in  their  unconsolidated  condition  form  soils.  If  they 
once  existed,  it  would  seem  strange  they  have  not  in  some  cases 
at  least  survived.  If  formed  in  soils,  why  should  they  not  be 
formed  in  secondary  rocks  where  the  conditions  are  apparently 
so  much  more  favorable? 

It  would,  to  the  writer  at  least,  seem  more  probable  that  the 
soluble  potash  of  soils  exists,  not  in  zeolitic  combination,  but 
in  some  of  the  numerous  decomposition  products  of  feldspar, 
nepheline,  scapolite,  etc.,  to  which  the  name  pinite  is  commonly 
applied.  Such  at  least  is  the  case  in  the  potash-rich  soils  of 
Maryland,  examined  by  R.  L.  Packard.2  It  is  possible  also 
that  it  may  exist  in  compounds  allied  to  glauconite. 

The  writer  has  elsewhere3  pointed  out  that,  particularly 
among  basic  rocks,  there  may  be  actually  a  larger  percentage 
of  matter  soluble  in  hydrochloric  acid  and  sodium  carbonate 
solution  in  rocks  ordinarily  designated  as  fresh,  than  in  the 
debris  resulting  from  their  decomposition.  This  fact  he  has 
since  emphasized  in  a  paper  read  at  the  December  (1896)  meet- 
ing of  the  Geological  Society  of  America,  and  from  which  the 
following  statements  are  drawn.  Rock -weathering,  it  must  be 
remembered,  is  in  the  majority  of  instances  accompanied  by  a 
leaching  process,  whereby  original  soluble  compounds,  or  new 
soluble  compounds  formed  during  the  process  of  decomposition, 
are  gradually  removed.  The  final  result  is  therefore,  as  already 
many  times  noted,  a  residue  consisting  of  the  least  soluble  con- 
stituents, and  which  forms  the  ordinary  surface  soil.  Even  in 
cases  where  the  actual  amount  of  soluble  matter  is  greatest  in 
a  soil,  the  apparent  excess  may  be  due  to  water  of  hydration 
and  to  the  large  amount  of  sesquioxide  of  iron,  the  latter  being 
practically  insoluble  in  meteoric  waters  so  long  as  there  is  a 
free  supply  of  oxygen,  though  readily  soluble  in  hydrochloric 
acid.  These  conclusions  are  based  upon  the  following  table,  in 
which  the  total  percentage  loss  on  ignition,  minus  the  ignition 
in  the  insoluble  residue,  is  tabulated  with  the  soluble  matter. 

1  Geologie  Experimental,  pp.  180  et  seq. 

2  Bull.  21,  Maryland  Agricultural  Experiment  Station,  1893. 

3  Bull.  Geol.  Soc.  of  America,  Vol.  VII,  1895,  p.  355. 


SOLUBLE   CONSTITUENTS   OF   ROCKS 


377 


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i2<-      f^oSW^S 

378  THE   REGOLITH 

(3)  Physical  Condition  of  the  Soil.  —  Chemically,  as  previ- 
ously noted,  a  soil  differs  from  the  parent  rock  in  the  amount 
of  leaching  it  has  undergone,  and  in  the  finely  comminuted  and 
more  or  less  decomposed  condition  of  its  particles.  There  are 
other  distinctions,  not  the  least  important  of  which  are  its  state 
of  loose  coherency  and  porous  condition  due  to  interstitial  air 
spaces.  It  has  been  estimated  by  Whitney l  that  the  approxi- 
mate number  of  grains  in  one  gramme  of  soil  varies  between 
2,000,000  and  15,000,000,  the  lowest  estimate  being  that  for  a 
sandy  soil  containing  only  some  4.77%  of  material  in  such  an 
extremely  fine  state  of  comminution  as  properly  to  be  classed 
as  clay,  while  the  highest  number  is  that  in  a  sub-soil  contain- 
ing some  32.45  %.  Our  interest  in  these  remarkable  figures  is 
still  further  heightened  when  we  are  called  upon  to  remember 
that  these  grains  are  not  in  actual  contact,  but  each  separated 
from  the  other  by  thin  films  of  moisture,  or,  in  a  dry  soil,  by 
actual  air  spaces.  That  such  spaces  exist  is  easily  proven  by  the 
fact  that  any  soil  may  be  greatly  diminished  in  bulk  by  pressure. 
The  amount  of  this  empty  space  is  naturally  quite  variable,  but 
it  is  estimated  to  constitute  on  an  average  some  50  %,  by  volume, 
of  the  soil.  That  is  to  say,  a  cubic  foot  of  soil,  in  its  natural 
condition,  contains  an  amount  of  space  between  its  grains,  filled 
by  air  or  water,  equal  to  one-half  the  entire  mass. 

These  extraordinary  figures  are  given,  not  merely  to  illustrate 
the  wonderful  degree  of  comminution  reached  in  rock-weather- 
ing, but  also,  and  what  is  of  more  importance  from  the  stand- 
point of  an  agriculturist,  the  amount  of  surface  exposed  to  the 
solvent  action  of  roots  and  percolating  waters.  Indeed,  it  has 
been  estimated  that  the  total  surface  areas  of  the  grains  in  a 
cubic  foot  of  soil  amounts,  on  the  average,  to  50,000  square  feet. 
The  amount  is  of  course  greater  in  a  fine  than  a  coarse  soil,  but 
in  any  case  sufficiently  large  to  enable  us  to  understand  how, 
under  the  ordinary  conditions  of  cultivation,  all  the  materials 
essential  to  plant  growth  may  in  a  brief  time  be  removed,  unless 
renewed  by  artificial  fertilizers. 

Further  than  this,  the  amount  of  space  between  the  grains 
is  of  very  great  importance  in  determining  the  circulation  of 
water  in  the  soil,  and  its  capacity  for  retaining  the  right  propor- 
tion essential  to  plant  growth  as  noted  later. 

The  experimental  work  of  late  years  goes  to  show  that  fertil- 
1  Bull.  No.  4,  U.  S.  Dept.  of  Agriculture,  Weather  Bureau. 


PHYSICAL   CONDITION  OF   SOILS  379 

ity  is  dependent  upon  these  physical  properties  perhaps  even 
more  than  upon  chemical  composition.  If  the  structure,  i.e. 
the  manner  of  arrangement  of  the  soil  particles,  is  such  as  to  be 
most  favorable  to  root  action  and  conservation  of  moisture,  there 
are  few  soils  but  may  be  made  fertile  by  proper  treatment,  even 
cannot  the  desired  physical  properties  be  imparted  by  artificial 
means.  A  soil  which  contains  too  large  a  proportion  of  fine 
clay  matter  may  hold  so  large  a  proportion  of  moisture  as  to 
be  quite  unsuited  for  cultivation  when  saturated,  and  become 
equally  unfitted  by  induration  when  dry.  A  light,  porous, 
sandy  soil  on  the  other  hand,  though  fertile  during  seasons  of 
abundant  precipitation,  parts  with  its  moisture  so  readily  as  to 
be  quite  barren  in  seasons  of  drought.  Porosity  and  capillarity, 
two  properties  dependent  wholly  on  the  size  and  shape  of  the 
soil  particles,  are  therefore  very  essential  items  in  this  consid- 
eration. Moisture  precipitated  in  the  form  of  rain  soaks  into 
the  ground  or  flows  off  upon  the  surface  in  varying  proportions, 
according  to  local  conditions,  an  open  porous  soil  naturally 
absorbing  more  rapidly  than  one  that  is  close  and  compact. 

When,  after  the  rain  ceases,  evaporation  sets  in  from  the 
surface,  the  water  which  has  soaked  in  is  brought  back  again 
in  part  by  capillarity,  though  a  part  escapes  through  leaching 
downward  beyond  the  reach  of  capillarity,  ultimately  coming 
to  the  surface  again,  at  lower  levels,  in  the  form  of  springs. 
The  capacity  of  a  soil  to  care  for  the  water  it  receives  from 
rains  is,  perhaps,  the  most  important  of  any  one  property. 
It  has  been  demonstrated  that  the  soils  of  the  semi-arid  regions 
of  the  West  will  produce  abundant  crops  of  wheat  and  corn, 
though  receiving  but  about  half  the  amount  of  water  from  rain- 
fall that  would  be  requisite  in  the  East.  This  is  accounted 
for  wholly  on  physical  grounds,  and  is  explained  as  follows  : : 
Water  falling  upon  a  perfectly  dry  soil  descends  very  slowly, 
and  indeed,  in  extreme  cases,  may  continue  to  fall  for  hours 
without  wetting  the  mass  for  more  than  a  few  inches  below  the 
surface,  while  it  will  be  absorbed  very  rapidly  by  a  soil  already 
wet,  but  not  saturated.  This  is  due  to  the  fact,  as  explained 
by  Whitney,  that  in  a  dry  soil  the  tension  or  contracting  power 
of  the  surface  of  the  water  is  greater  than  the  attraction  of  the 
soil  grains.  If,  on  the  other  hand,  there  is  any  appreciable 

1  Conditions  in  Soils  of  the  Arid  Region,  by  Milton  Whitney,  Yearbook 
U.  S.  Dept.  of  Agriculture,  1894. 


380  THE   REGOLITH 

amount  of  moisture  in  the  soil,  the  tension  of  the  water  sur- 
face will  cause  it  to  contract  and  pull  the  water  from  above 
into  the  sub-soil.  It  follows,  then,  that  the  water  of  rains  fall- 
ing in  semi-arid  regions  would  not  penetrate  into  the  dry  sub- 
soil, until  the  overlying  portions  had  become 'successively  so 
far  saturated  that  they  could  no  longer  hold  the  water  back, 
and  it  would  pass  downward,  therefore,  very  gradually  into  the 
lower  depths,  saturating,  or  nearly  saturating,  each  successive 
depth  as  it  progressed.  Unless,  then,  as  rarely  happens  in  this 
region,  the  rainfall  was  so  great  and  so  continuous  as  to  saturate 
the  soil  to  a  considerable  depth,  the  whole  supply  of  moisture 
absorbed  would  remain  within  a  short  distance  of  the  surface, 
either  immediately  within  reach  of  plant  roots,  or  where  it  can 
be  brought  upwards  once  more  by  capillarity  when  evaporation 
from  the  surface  begins.  With  a  continuously  wet  sub-soil, 
however,  as  in  the  East,  a  very  considerable  portion  of  the 
water  passes  at  once  to  depths  beyond  the  reach  of  roots  or 
capillary  attraction,  and  is,  so  far  as  our  present  considerations 
are  concerned,  completely  lost  until,  in  the  course  of  nature's 
endless  cycle,  it  shall  once  more  be  returned  as  rain.  Within 
certain  limits,  a  small  rainfall,  equitably  distributed,  is  more 
advantageous  to  agriculture  than  are  the  heavier  precipita- 
tions which  characterize  the  Atlantic  slopes  of  the  American 
continent. 

The  capacity  of  soils  for  moisture  has  been  the  subject  of 
experiment,  and  is  found  to  vary  widely,  being  naturally  largely 
dependent  upon  the  size  of  the  individual  particles  and  the  con- 
sequent amount  of  interspace.  Whitney  states 1  that  sub-soils 
of  Maryland  truckland  having  45  %  of  interspace  will  hold  but 
22.41  %  by  weight  of  water,  when  this  space  is  completely  filled. 
The  sub-soil  of  the  Helderberg  limestone,  having  65  %  of  space, 
will  hold  41.22%.  King2  gives  the  following  table  to  show 
the  actual  amount  of  water  held  by  field  soils  when  their  sur- 
faces are  only  11  inches  above  standing  water,  this  water  having 
been  lifted  into  them  by  capillarity :  — 

1  Some  Physical  Properties  of  Soils,  Bull.  No.  4,  U.  S.  Dept.  of  Agriculture, 
Weather  Bureau,  1892. 

2  The  Soil,  p.  159. 


KINDS   OF   SOILS 


381 


SOIL 

PER  CENT 
OF  WATER 

POUNDS  OF 
WATER 

INCHES  OK 
WATER 

Surface  foot  of  clay  loam  contained  .     .     . 

32.2 

23.9 

4.59 

Second  foot  of  reddish  clay  contained  . 

23.8 

22.2 

4.26 

Third  foot  of  reddish  clay  contained 

24.5 

22.7 

4.37 

Fourth  foot  of  clay  and  sand  contained 

22.6 

22.1 

4.25 

Fifth  foot  of  fine  sand  contained  .... 

17.5 

19.6 

3.?7 

Total     

110.5 

21.24 

According  to  Meister,    different   soils   show   water-holding 
capacities  as  follows  : 1  - 


PER  CENT 

PER  CENT 

KIND  OF  SOIL 

OF  WATER 

KIND  OF  SOIL 

OF  WATER 

IMBIBED 

IMBIBED 

Clay     

50.0 

Chalk     

49.5 

Loam  

60.1 

Gyseous  

52.4 

Humus     

70.3 

Sandy  (82  %  sand)    .     .     . 

45.4 

Peat     

63.7 

Sandy  (64%  sand)    .     .     . 

65.2 

Garden 

69.0 

Pure  quartz  sand. 

46.4 

Lime    

59.9 

(4)  The  Weight  of  Soils.  —  This  is  dependent  upon  (1)  the 
character  of  the  particles  composing  the  soil  and  (2)  their 
degrees  of  compactness.  The  figures  given  below  are  those  of 
Schubler.2 


WEIGHT  PER  CUBIC  FOOT  IN  POUNDS,  OF  VARIOUS  SOILS 

Dry  siliceous  or  calcareous  sand 

Half  sand  and  half  clay 

Common  arable  soil 

Heavy  clay 

Garden  mould,  rich  in  vegetable  matter   .     . 

Peat  soil  


110 
96 

80-90 
75 
70 
30-50 


(5)  Kinds  and  Classification  of  Soils.  —  Being  derived  from 
rocks  of  all  kinds  and  under  greatly  varying  conditions;    in 

i  Handbook  of  Experiment  Station  Work,  U.  S.  Dept.  of  Agriculture,  1893, 
p.  317. 

*  Handbook  of  Experiment  Station  Work,  U.  S.  Dept.  of  Agriculture,  1893, 

p.  315. 


382  THE   REGOLITH 

almost  infinitely  variable  conditions  of  comminution,  decay,  and 
proportional  amounts  of  their  various  constituents,  no  hard  and 
fast  lines  for  soil  classification  can  be  laid  down.  All  things 
considered,  they  are  best  classed  with  the  regolith  of  which  they 
form  a  part,  the  general  divisions  of  which  are  given  in  tabular 
form  011  p.  299.  We  thus  have  the  primary  divisions  of  seden- 
tary.and  transported  soils,  accordingly  as  they  have  been  formed 
in  place,  or  transported.  Each  of  these  is  again  subdivided 
according  to  the  agencies  involved  in  its  transportation  or 
original  formation. 

Many  varietal  names  have  been  applied  to  soils,  but  as  a  rule 
in  so  loose  and  ill-defined  a  manner  as  to  give  them  only  a  very 
general  significance.  A  common  practice  is  to  name  one  of 
sedentary  origin  according  to  the  rock  from  which  it  was  de- 
rived, as  granite  soil,  limestone  soil,  etc.  Transported  soils,  on 
the  other  hand,  are  often  designated  either  by  the  agencies  in- 
volved in  transportation,  as  glacial,  or  ceolian  soils,  their  position, 
as  terrace  soils,  or  their  physical  or  chemical  characteristics,  as 
sandy  or  clayey  soils.  A  loam  is  usually  defined  as  an  admixture 
of  sand  and  clay  with  more  or  less  organic  matter,  a  clayey 
loam  being  one  in  which  clay  predominates  and  a  sandy  loam 
one  in  which  sand  prevails.  The  terms  peat,  muck,  loess,  marl, 
etc.,  have  been  already  sufficiently  defined.  Local  names  indi- 
cative of  suitability  for  particular  crops,  or  sometimes  of  doubt- 
ful or  obscure  meaning,  are  frequently  met  with.  The  bluegrass 
soils  of  central  Kentucky  are  limestone  residuals  celebrated  for 
the  luxuriant  growths  of  Poa  pratensis  which  they  bear.  The 
red  "  buckshot "  soils  of  the  Yazoo  bottoms,  Louisiana,  are  stiff 
clayey  alluvial  soils  mottled  with  ferruginous  spots. 

Many  names  indicative  of  mode  of  formation  have  already 
received  attention,  but  a  few  others  may  be  here  noted.  The 
names  Endogeneous  and  Exogeneous  have  been  proposed  for 
soils  formed  in  place  (sedentary)  or  derived  from  other  sources 
(transported).  It  is  presumably  scarcely  necessary  to  remark 
that  such  terms  are  quite  inapplicable  and  inappropriate. 

The  name  regur  is  locally  applied  to  a  fine  dark  argillaceous 
soil  particularly  suited  for  cotton  growing  and  which  has  a  wide 
areal  distribution  throughout  southern  India.  Its  origin  ap- 
pears to  be  mainly  subaerial,  though  a  part  of  the  material  so 
called  is  undoubtedly  alluvial.  The  material  is  highly  plastic 
when  wet,  and  expands  and  contracts  to  a  remarkable  degree 


KINDS   OF   SOILS 


383 


under  varying  conditions  of  moisture  and  dryness.  This  soil 
is  very  retentive  of  moisture  and  rarely  requires  to  be  irrigated 
artificially.  It  is,  as  a  rule,  of  great  fertility  and  of  wonderful 
lasting  powers,  it  being  stated  that  in  some  localities  it  has 
borne  crops  for  2000  consecutive  years,  without  the  aid  of  ma- 
nures. In  depth  this  soil  is  rarely  over  6  to  8  feet.  The  follow- 
ing analyses  show  the  chemical  character  of  the  regur  (from 
near  Seoni)  on  the  surface  and  at  depths  of  (An)  5  feet  and 
(Bn)  3  feet  below.  The  analyses  A,  like  those  given  on 
p.  306,  are  instructive  as  showing  the  large  increase  in  the 
amount  of  lime  from  the  surface  downward.  Although  not  so 
stated,  the  slight  differences  in  Bi  and  Bn  are  probably  due  to 
the  lesser  depth  below  the  surface  from  which  Bn  was  taken. 


l 

L 

I 

I 

CONSTITUENTS 

I 

II 

I 

II 

Insoluble  matter  
Organic  matter  
Water  
Oxide  of  iron  .  ... 

62.7% 
9.2 
8.4 
11.0 

47.61  % 
8.4 
7.6 
15.9 

62.  8% 
9.0 
8.2 
10.9 

63.7% 
8.7 
6.5 
11.8 

Alumina  .  

7.5 

8.6 

7.6 

8.4 

Carbonate  of  lime  

1.2 

11.89 

1.5 

1.3 

100.00% 

100.00% 

100.00% 

100.00% 

In  many  cases  this  regur  is  derived  directly  from  basaltic 
rocks,  by  surface  decomposition  in  situ,  whilst  other  varieties 
were  derived  from  other  aluminous  rocks,  or  are  alluvial  depos- 
its in  river  valle}^s,  lakes,  lagoons,  and  marshes.  The  dark 
color,  as  is  usual,  is  due  to  the  presence  of  organic  matter.1 

The  term  sub-soil  is  applied  to  that  portion  of  the  regolith 
which  immediately  underlies  the  soil  proper,  and  from  which 
it  differs  mainly  in  compactness,  and  the  lesser  amount  of  oxi- 
dation and  decomposition  it  lias  undergone.  In  a  soil  which 
has  never  been  cultivated,  the  sub-soil  may  pass  gradually  up- 
ward into  the  soil  without  distinct  lines  of  demarcation.  Pro- 
longed cultivation  may,  however,  have  so  thoroughly  oxidized 
and  physically  altered  the  superficial  portions  down  to  the  limit 
of  plough  and  root  action,  as  to  bring  about  a  very  marked  cliff  er- 

i  Manual  of  the  Geology  of  India,  2d  ed.,  by  R.  D.  Oldham,  1893,  p.  411. 


384  THE    REGOLITH 

ence,  both  in  color  and  texture,  as  well  as  in  actual  composition. 
At  times  the  sub-soil  becomes  so  thoroughly  compacted  as  to  be 
almost  impervious,  forming  a  so-called  hardpan. 

(6)  The  Color  of  Soils.  —  The  color  of  soils  is  due  mainly  to 
carbonaceous  matter  and  iron  oxides.  To  the  first  are  due  the 
dark  gray  to  black  colors  characteristic  of  prairie  and  swamp 
soils.  To  iron  oxides  are  due  the  buff,  yellow,  ochreous-brown, 
and  red  hues,  the  source  of  the  oxides  being  mainly  the  silicate 
minerals  from  whence  the  soils  were  derived.  It  not  infre- 
quently happens,  as  abundantly  demonstrated  in  the  southern 
Appalachian  states,  that  it  is  possible  in  passing  over  any  sec- 
tion of  the  country  to  designate  with  a  fair  degree  of  accuracy 
the  lithological  nature  of  the  underlying  rocks  from  the  color 
of  the  residual  soils,  even  though  the  rocks  themselves  may 
be  wholly  ob'scured  by  decomposition  products.  In  such  cases 
rocks  rich  in  iron  silicates,  like  hornblende  and  augite,  give 
rise  to  bright  red  soils,  while  those  poor  in  these  constituents 
yield  soils  of  a  gray  or  slightly  yellowish  hue.  Much,  however, 
apparently  depends  on  extent  of  decomposition  and  on  climatic 
conditions,  as  noted  below. 

One  of  the  most  striking  features  of  the  landscape  observed 
in  travelling  southward  along  the  Appalachian  belt  is  the 
abrupt  transition  in  color  of  the  soil,  as  the  limit  of  glacial 
action  is  past.  Within  the  glaciated  area,  except  where  de- 
rived directly  from  rocks  themselves  highly  colored,  like  the 
Triassic  sandstones,  the  soils  are  everywhere  dull  in  color, 
some  shade  of  gray,  drab,  or  brown.  South  of  this  limit  ochre- 
ous-red  and  yellowish  prevail.  Along  the  line  of  the  Virginia 
Midland  railway,  south  of  Washington,  these  colors  prevail  in 
hues  of  astonishing  brilliancy.  Although  the  soils  throughout 
the  region  are  residual,  their  colors  seem  in  many  cases  quite 
independent  of  the  kind  of  rock  to  which  they  owe  their  origin. 
Granite,  gneiss,  schist,  or  basic  trappean  rocks  alike  give  rise  to 
red  and  yellow  highly  tenacious  soils  of  such  depth  and  brill- 
iancy of  color  that  every  gully,  ravine,  and  roadway  stands  out 
against  the  green  background  of  the  landscape,  as  though 
painted  by  some  Titanic  hand  with  brushes  dipped  only  in 
yellow,  red,  and  vermilion  ochres.  These  contrasts  are  par- 
ticularly striking  in  the  early  summer  and  directly  after  a 
rain.  But  he  who  wishes  to  admire  had  best  do  so  from  his 
window,  and  without  too  much  attention  to  detail. 


THE   COLOR   OF   SOILS  385 

The  soil  is  plastic  and  adherent  to  an  intolerable  degree. 
The  grass  forms  no  compact  sod,  as  in  the  North,  and  as  a  re- 
sult the  walls  of  houses,  fences,  feet,  legs  and  clothes  of  pedes- 
trians become  uniformly  stained  a  dirty  ochreous  color  equally 
trying  to  the  housewife  and  to  ploughman. 

The  cause  of  this  color  variation  has  been  the  subject  of 
speculation  by  Professors  Crosby,1  Dana,2  Russell,3  and  others. 
So  far  as  our  knowledge  now  extends,  it  is  apparent,  as  first 
stated  by  Crosby,  that  the  difference  is  due  to  a  spontaneous 
dehydration  which  takes  place  in  the  warmer  regions,  whereby 
the  hydrous  sesquioxides  of  the  type  of  limonite  and  gothite 
are  converted  into  the  less  hydrated  or  anhydrous  forms  tur- 
gite  and  hematite  with  corresponding  changes  in  color  from 
yellow  or  brown  to  red. 

This  view  is  rendered  the  more  plausible  from  the  fact  that 
the  most  brilliant  hues  are  entirely  superficial,  and  below  the 
surface  gradually  fade  out  into  brown  and  yellow  or  even  gray 
hues.  Such  a  transition  may  be  observed  in  any  fresh  road 
cut,  but  quickly  becomes  obscured  by  the  washing  down  of  the 
deeply  colored  material  from  the  higher  levels.  Sometimes 
the  brilliant  red  will  be  found  a  mere  wash,  but  a  fraction  of 
an  inch  in  thickness,  or  again  it  penetrates  to  the  depth  of 
a  foot  or  more  before  giving  way  to  more  modest  hues.  In 
such  cases  the  brilliant  colors  will  be  found  to  have  penetrated 
deepest  along  joint  lines,  or  the  more  porous  portions,  leaving 
the  intervening  compact  masses  of  more  sombre  hue. 

In  discussing  this  matter,  there  is,  however,  one  point  that  we 
should  not  overlook,  although  its  importance  seems  not  to  have 
been  fully  realized  by  the  authorities  quoted,  and  that  is,  a 
change  in  color  not  due  alone  to  a  change  in  the  conditions  of 
the  iron,  but  to  the  relative  greater  abundance  of  this  constitu- 
ent in  the  upper  portions.  The  iron  oxides,  as  already  noted, 
owing  to  their  less  soluble  nature  accumulate  in  the  residues, 
and  as  a  rule,  the  more  thorough  the  decomposition  the  greater 
the  proportional  amount  of  iron.  A  small  percentage  of  free 
oxide  disseminated  throughout  a  relatively  large  amount  of 
detritus  imparts  but  little  color  ;  the  more  iron,  the  more  color. 

1  Proc.  Boston  Society  of  Natural  History,  1885,  p.  219,  and  Technological 
Quarterly,  Vol.  IV,  1891,  p.  30. 

2  Am.  Jour,  of  Science,  Vol.  XXXIX,  1890,  pp.  317-319. 
8  Bull.  No.  52,  U.  S.  Geol.  Survey,  1889. 

2c 


386  THE   REGOLITH 

The  residue  from  the  Medford  diabase  described  on  p.  220  is 
of  a  deep  brown  color,  as  a  whole,  but  the  finest  silt  washed 
from  it  is  several  shades  brighter,  of  a  dull  ochreous  red.  Had 
the  entire  mass  decomposed  to  the  condition  of  this  silt,  we 
might  expect  it  to  have  the  same  color.  This  change  in  color, 
due  to  increased  proportional  amounts  of  iron  oxides,  is  particu- 
larly marked  in  limestone  residuals  where  the  original  rock  may 
contain  merely  traces  of  free  oxides,  or  ferruginous  silicates. 
Neumayer  has  shown1  that  the  snow-white  Karst  limestones 
contain  only  some  0.044  %  of  ferruginous  silicates  which  them- 
selves carry  20  %  of  iron  oxides.  Yet  the  residual  soil  left  by 
the  decomposition  of  this  limestone  is  of  so  pronounced  a  color, 
as  to  have  long  ago  received  the  name  terra  rossa  or  red  earth. 
Other  things  being  equal,  brilliancy  of  color  may  then  be 
regarded  as  (1)  indicative  of  advanced  decomposition,  and 
(2)  of  geological  antiquity. 

(7)    The  Age  of  Soils.  —  No  sooner  were  the  first  rocks  pushed 
above  sea-level  than  the  various  agencies  described  under  the 

head  of  weathering  began  the 
work  of  disintegration,  de- 
composition, and  transporta- 
tion. Of  this  we  have  ample 
proof  in  the  entire  series  of 
sedimentary  rocks  extending 
from  the  Archaean  down  to 
the  most  recent  and  which 
are  but  the  reconsolidated 
FIG.  38.  —  Trunk  of  tree  still  standing  in  residues  of  preexisting  masses. 


soil  of  Carboniferous  age.     a,  bed-rock  ;    ^hat    guc]1    a  breaking   down 
b,  under  clay  or  ancient  soil;   c,  coal;  11-1  i       ,  •  *• 

d,  bedded  rock;  e,  fossil  tree.  resulted  in  the  production  of 

soils  is  a  fair  inference,  though 

we  have  no  absolute  evidence  of  land  plants  and  hence,  a 
priori,  of  soils,  before  the  beginning  of  the  Upper  Silurian 
period,  when  plants  of  the  lycopod  type  appeared.  Such  soils, 
as  soils,  have,  however,  long  since  disappeared  in  the  never- 
ending  cycle  of  change  and  it  is  not  until  we  reach  the  Car- 
boniferous period  that  we  meet  with  soils  which  have  been 
preserved  in  place  and  in  recognizable  form  even  to  the  present 
day.  Even  here  induration  and  partial  metamorphism  has 
rendered  them  no  longer  fitted  for  the  support  of  plant  life, 

1  Verhandl.  k.  k.  Geol.  Reichsandstalt,  1875-76,  p.  50. 


THE   AGE    OF   SOILS  387 

but  that  they  once  did  so  serve  is  amply  proven  by  the  occa- 
sional finding  of  still  erect,  though  fossil,  trunks  with  roots 
buried  in  their  native  soil,  as  they  grew  in  the  marshes  and 
woodlands  of  the  coal  period.  But  as  to  the  time  of  the  begin- 
nings of  the  formation  of  such  soils  as  still  retain  their  soil 
characteristics,  we  have  not  in  all  cases  reliable  data.  Those 
which  are  but  the  unconsolidated  sediments  of  recent  geological 
time,  like  those  of  the  eastern  shore  of  Maryland,  the  loess  and 
alluvium  of  the  Mississippi  valley,  or  the  swamp  and  glacial 
soils  of  the  north  and  east  may,  of  course,  be  located  with  a 
reasonable  amount  of  accuracy.  But  as  for  the  residual  soils, 
those  which  result  from  the  breaking  down  in  place  of  rock 
masses,  we  can  only  say  that  they  must  be  younger  than  the 
rocks  from  which  they  were  derived.  The  writer  has  shown 
that  the  granite  soils  of  the  District  of  Columbia  are  post- 
Cretaceous  ;  in  other  parts  of  the  Piedmont  plateau  of  Mary- 
land, they  may  be  post-Tertiary.  In  but  few  instances,  as  at 


Poor  Soil 


FIG.  39. 


Medford  in  Massachusetts,  have  we  evidence  of  any  consider- 
able amount  of  soil  formation  by  decomposition  and  disintegra- 
tion since  the  close  of  the  glacial  period.  Obviously  the  older 
a  residual  soil,  the  greater  the  amount  of  decomposition  and 
leaching  it  will  have  undergone  and  the  less  will  it  resemble 
the  parent  rock.  Where  horizontally  lying  strata  of  varying 
character  have  successively  undergone  decomposition  and  a  loss 
of  their  soluble  constituents,  the  resultant  soil  must  periodically 
vary  according  to  the  nature  of  the  rock  undergoing  decompo- 
sition and  the  inherited  characteristics  handed  down  from  the 
strata  earlier  decomposed.  In  such  a  case  as  that  here  figured, 
we  have  a  residual  soil  containing  the  least  soluble  constituents 
of  the  hundreds  of  feet  of  dissolved  and  disintegrated  rock  which 


388  THE   KEGOLITH 

once  extended  across  the  entire  country  becoming  commingled 
with  that  now  undergoing,  in  its  turn,  the  soil-making  process. 
Such  a  soil  may,  therefore,  in  extreme  cases,  contain  materials 
of  all  ages  from  the  first  product  of  disintegration  of  the  upper- 
most strata,  which  may  have  been  Carboniferous,  to  that  which 
formed  to-day,  and  may  be  Cambrian. 

It  is,  of  course,  true  that  through  the  erosive  action  of  water 
these  soils  are  continually  losing  their  finer  silt  and  clay-like 
particles,  it  may  be  almost  as  fast  as  formed,  especially  in  hilly 
regions,  and  that  as  the  soil  drops  lower  and  lower  in  the  geo- 
logical horizons  indicated,  it  becomes  more  and  more  impover- 
ished in  those  constituents  derived  from  the  upper  beds.  But 
as  to  what  proportion  of  the  material  of  one  horizon  is  handed 
down  to  become  admixed  with  that  from  the  rocks  below,  we 
have  no  means  of  judging,  and  in  fact  it  must  be  ever-varying. 

The  matter  of  the  geological  age  of  any  soil,  or  the  age  of 
the  rocks  from  which  it  was  derived,  is  therefore  of  only  very 
general  interest,  and  may  w^ell  be  dismissed  here.  The  attempt 
which  has  been  made  by  another  writer1  to  discriminate  or 
classify  soils  according  to  the  geological  horizons  of  the  rocks 
from  which  they  were  derived,  is  believed  by  the  present  writer 
to  be  futile  and  wholly  inexpedient. 

No  attempt  should  be  made,  as  has  been  done  by  at  least  one 
writer,  to  state  the  character  of  soil  that  may  arise  from  the 
weathering  of  any  particular  class  of  rocks,  since  much  depends 
upon  the  extent  to  which  weathering  has  been  carried.  The 
ultimate  product  of  weathering  of  rocks  of  any  but  the  purely 
siliceous  type  is  a  more  or  less  ferruginous  clay,  which  may 
be  contaminated  or  admixed  with  coarser,  foreign  particles. 
It  is  the  extent  of  decomposition,  more  than  its  lithological 
derivation,  that  determines  both  the  chemical  composition  and 
physical  characteristics  of  any  soil. 

Rocks  of  essentially  the  same  type  so  far  as  composition  is 
concerned,  regardless  of  structural  modifications  induced  by 
metamorphism,  have  been  formed  and  re-formed  throughout 
every  period  of  the  earth's  history,  and  the  attempt  made  to 
classify  those  of  igneous  origin  from  the  standpoint  of  geologi- 
cal age  has  invariably  resulted  in  failure.  As  has  been  already 
indicated,  the  greater  portion  of  the  granitic,  gneissic,  or  highly 
metamorphosed  crystalline  schists  and  calcareous  rocks  belong 
1  See  Stockbridge's  Rocks  and  Soils,  p,  12, 


EFFECT   OF   PLANT   AND   ANIMAL   LIFE  389 

either  to  the  Archaean  or  older  Palseozoic  formations,  but  this 
merely  because  they,  being  older,  have  been  longer  subjected  to 
metamorphosing  agencies,  and  not  because  in  themselves  they 
possess  essential  differences.  It  is  true  that  some  authorities 
lay  stress  on  the  supposed  abundance  of  animal  remains  in  cer- 
tain Palseozoic  formations,  but  no  one  but  the  veriest  amateur 
would  now  dream  of  attempting  to  discriminate  between  either 
igneous  or  aqueous  rocks  of  the  same  nature,  but  of  different 
geological  ages,  on  purely  chemical  grounds. 

It  is  a  fact,  however,  that  within  certain  climatic  limits,  the 
rocks  of  any  one  horizon  may  impart  such  characteristics  to  a 
residual  soil  as  shall  render  it  adapted  to  plant  growth  of  a 
particular  kind.  Thus,1  throughout  the  central  portion  of 
Kentucky,  where,  within  the  distance  of  a  few  miles,  rocks 
occur  of  several  distinct  geological  horizons,  each  bearing  its 
mantle  of  residual  soil,  each  horizon  may  be  traced  for  long 
distances,  though  the  rocks  themselves  are  wholly  obscured, 
merely  by  the  character  of  its  forest  growth.  » 

(8)  Soils  as  affected  by  Plant  and  Animal  Life.  —  There  are 
various  forms  of  animal  and  plant  life  the  action  of  which  is 
worthy  of  note  in  connection  with  the  subject  of  decomposition  ; 
but  since  it  is  probable  that  they  are  of  greater  efficiency  in 
promoting  changes  in  soils  once  formed  than  in  bringing  about 
the  preliminary  rock  disintegration,  their  consideration  has 
been  left  to  form  a  portion  of  the  present  chapter. 

Ants,  by  means  of  their  numerous  borings,  penetrating  at 
times  to  depths  of  many  feet,  bring  about  not  merely  a  rear- 
rangement of  soil  particles  through  a  transfer  of  materials  from 
lower  to  higher  levels,  but  also  a  condition  of  porosity  whereby 
air  and  water  gain  access  to  the  deeper  lying  portions,  there 
to  promote  further  chemical  and  physical  changes. 

Naturally  these  insects  limit  their  work  to  dry  and  light 
soils,  where  their  operations  may  be  compared  with  that  of 
earthworms  whose  operations  are  confined  to  moist  ones. 
Shaler  has  calculated2  that  over  a  certain  field  in  Cambridge 
(Massachusetts)  the  ants  have  made  an  average  transfer  of  soil 
matter  from  the  depths  to  the  surface  sufficient  to  form  a  layer 
each  year  of  at  least  a  fifth  of  an  inch  over  the  entire  four  acres 
under  observation.  He  further  mentions  a  curious  effect  aris- 

1  As  the  writer  is  informed  by  Mr.  J.  R.  Proctor. 

2  12th  Ann.  Rep.  U.  S.  Geol.  Survey,  1890-91,  p.  278. 


390  THE   REGOLITH 

ing  from  the  interference  of  the  ants  with  the  original  condi- 
tions, in  the  separation  of  the  finer  from  the  coarser  particles. 
In  certain  parts  of  New  England  where  sandy  soils  had  laid 
for  a  long  time  uncultivated,  fields  were  covered  to  a  depth  of 
some  inches  with  a  layer  of  fine  sand  without  pebbles  larger 
than  the  head  of  a  pin,  while  below  the  level  of  perhaps  a  foot 
the  soil  was  mainly  pebbles,  with  very  little  finer  material. 
This  condition,  it  is  argued,  was  brought  about  by  the  tens  of 
thousands  of  ants  which  each  year,  over  every  acre,  in  the 
process  of  building  their  dwelling  brought  up  the  finer  material 
and  deposited  it  in  the  form  of  a  mound  about  the  surface 
opening,  leaving  behind  the  coarser  particles,  too  heavy  for 
them  to  move.  The  common  black  and  brown  ants  of  the 
United  States  (Formica  exsectoides^)  build  upon  the  surface 
mounds  not  infrequently  from  1  to  2  feet  in  height  and  3  to 
5  feet  in  diameter  and  which  are  composed  of  materials  brought 
up  from  below  intermingled  with  twigs  and  shreds  of  bark  and 
leaves  from  the  surface.  Shaler  calculates  the  mass  of  some 

of  these  mounds  as  equal 
to  2  cubic  yards.  Being 
of  unconsolidated,  loosely 
coherent  material,  such 
are  constantly  being  de- 

FIG.  40. -Effects  of  ant-hills  on  soils.   a«,sand     Sraded   ^   wind   and    rail! 
accumulated  in  hill;    bb,  material    washed     and     their      particles     dis- 

Sl°PeS>  mingled  with  vegetable  tributed  over  the  sur- 
rounding surface.  "Where 
these  structures  are  numerous,  as  they  are  in  certain  districts 
in  the  United  States,  by  their  constant  deposits  of  matter  on 
the  surface  of  the  ground,  they  bury  a  good  deal  of  vegeta- 
ble waste  in  the  soil ;  at  the  same  time  the  animals  are  con- 
stantly conveying  into  the  earth  large  quantities  of  organic 
matter  which  serves  them  as  food,  and  the  waste  of  this, 
including  the  excreta  of  the  animals  themselves,  is  of  con- 
siderable importance  in  the  refreshment  of  the  soil."  The 
geological  efficacy  of  insects  of  this  and  other  types  is  un- 
doubtedly greater  in  warmer  climes,  where  not  only  are  they 
found  in  greater  abundance,  but  their  period  of  activity  ex- 
tends over  a  larger  portion  of  the  year.  Messrs.  Mills  and 
Branner,  as  already  noted,  are  inclined  to  lay  considerable 
stress  on  the  work  of  ants  and  termites  in  bringing  about  soil 


EFFECT    OF   PLANT   AND   ANIMAL   LIFE  391 

changes  and  rocks  decomposition  in  Brazil.  Branner  states 
that  in  some  parts  of  the  Amazon  valley,  of  Minas  Goyaz  and 
Matto  Grosso,  the  soil  "  looks  as  if  it  had  been  literally  turned 
inside  out  by  the  burrowing  of  ants  and  termites."  The  species 
popularly  known  as  saubas  excavate  chambers  and  build  gal- 
leries which  are  frequently  from  50  to  100  feet  long,  from  10 
to  20  feet  across,  and  from  1  to  4  feet  high,  and  contain  tons  of 
earth.  The  white  ants  or  termites,  like  the  true  ants,  burrow 
extensive  channels  in  the  ground,  and  build  up  huge  nests 
upon  the  surface  from  the  size  of  which  one  may  gain  some 
idea  of  the  extent  of  the  underground  galleries.  In  the  region 
extending  from  the  state  of  Parana  to  north  of  the  Amazon 
and  along  the  upper  Paraguay  in  Matto  Grosso  may  be  seen 
places  where  the  nests  are  so  close  together  that  one  can  al- 
most walk  upon  them  for  several  hundred  yards  at  a  time, 
while  no  one  of  the  nests  is  more  than  10  feet  from  another 
over  many  acres  of  ground.  Such  nests  vary  in  size  from  1 
to  12  feet  in  height  and  1  to  10  feet  in  diameter,  and  do  not 
seem  to  be  confined  to  any  particular  kind  of  country,  though 
especially  noticeable  in  the  interior  and  timberless  regions.  The 
constant  transference  of  such  quantities  of  soil  from  below  to 
the  surface,  and  of  organic  matter  from  the  surface  downward, 
cannot  fail  to  bring  about  marked  changes  in  its  physical  as 
well  as  chemical  condition,  while  at  the  same  time  affording 
passageways  for  air  and  meteoric  waters,  as  already  noted. 

Certain  animals,  like  the  crayfish,  have  likewise  a  habit  of 
burrowing  in  the  ground,  though  as  they  are  wholly  subterra- 
nean or  aquatic  in  their  nature,  the  results  are  less  conspicuous 
to  the  casual  observer.  In  searching  for  their  food,  these  ani- 
mals bore  numerous  horizontal  channels  or  galleries  some- 
times an  inch  or  so  in  diameter  and  extending  for  many  feet, 
usually  ending  in  an  upward  shaft  extending  to  the  surface, 
or  at  the  margin  of  a  pond  or  stream.  These  form  natural 
drainage  channels  and  allow  a  more  ready  access  of  air,  con- 
verting what  might  under  other  conditions  be  a  heavy,  clayey 
or  even  marshy  soil,  unfit  for  cultivation,  into  one  light  and 
fertile. 

By  burrowing  through  dams  and  embankments,  they  have, 
however,  in  some  instances  so  weakened  these  structures  as  to 
cause  them  to  give  way,  whereby  large  districts  have  become 
inundated  and  for  a  time  rendered  unfit  for  cultivation. 


392  THE   REGOLITH 

Probably  none  of  the  forms  of  animal  life  thus  far  mentioned 
produce  such  wide-spread  and  beneficial  results  as  have  been 
ascribed  by  Darwin 1  to  the  common  earthworm,  the  angleworm 
of  the  New*  England  disciples  of  Izaak  Walton.  These  insig- 
nificant creatures,  as  is  well  known,  burrow  in  the  moist  rich 
soil,  and  derive  their  nourishment  from  the  organic  matter  it 
may  contain.  In  order,  however,  to  obtain  this  comparatively 
small  amount  of  nutritive  matter,  they  devour  the  earth  with- 
out any  selective  power,  and  pass  it  through  their  alimentary 
canals,  rejecting  the  remainder,  which  nearly  equals  in  bulk 
that  first  taken  in.  The  nufnerous  holes  made,  while  in  part 
perhaps  to  afford  passage  to  the  surface,  are  mainly  excavated 
in  this  process  of  soil  eating  and  actually  represent  the  amount 
of  material  which  the  worms  have  passed  through  their  diges- 
tive systems. 

Darwin  states  that  in  certain  parts  of  England  these  worms 
bring  to  the  surface  every  year,  in  the  form  of  excreta,  more  than 
10  tons  per  acre  of  fine  dry  mould,  "  so  that  the  whole  superficial 
bed  of  vegetable  mould  passes  through  their  bodies  in  the  course 
of  every  few  years."  By  actually  collecting  and  weighing  the 
excretions  deposited  on  a  small  area  during  a  given  time,  he 
found  that  the  rate  of  accumulation  was  at  the  rate  of  two- 
tenths  of  an  inch  a  year,  or  an  inch  in  every  five  years.  The 
importance  of  these  worms,  then,  both  as  mellowers  of  the  soil 
and  as  levellers  of  inequalities  —  by  burying  stones  and  filling 
hollows  —  is  therefore  very  great,  and  we  cannot  afford  to 
overlook  it  here. 

While  the  main  influence  of  the  worm  is  manifested  in  a 
mellowing  by  burrowing  and  a  transfer  of  material  from  a 
lower  to  a  higher  level,  they  bring  about  a  slight  admixture 
of  organic  matter  through  a  habit  of  coming  to  the  surface  at 
night  time,  and  dragging  down  into  their  burrows  small  shreds 
of  leaves  and  grass,  which,  taken  into  account  in  connection  with 
the  excrementitious  matter  of  the  worms  themselves,  must  tend, 
though  it  may  be  ever  so  slightly,  to  enrich  the  soil.  The  sub- 
ject should  not  be  dropped  without  referring  to  the  abundance 
of  these  worms,  which  in  England  has  been  estimated  as  at  the 
rate  of  53,767  to  each  acre  of  garden  land,  and  about  one  half 
that  number  for  pasture  land.  It  is  scarcely  necessary  to  re- 
mark that  their  distribution  is  very  unequal  throughout  the 
1  The  Formation  of  Vegetable  Mould. 


EFFECT   OF   PLANT   AND   ANIMAL   LIFE  393 

world,  and  that  in  dry  sandy  regions  they  are  almost,  if  not 
wholly,  unknown. 

In  northern  temperate  climates,  such  as  that  of  New  England, 
and  particularly  where  the  soil  is  of  a  clayey  nature  like  the 
ground  moraine,  the  burying  action  of  the  earthworm,  as  de- 
scribed above,  may  be  wholly  overcome  through  the  heaving 
action  of  frost.  Every  farmer  boy  who  has  been  condemned 
to  pick  the  drift  boulders  from  a  field  knows  through  bitter 
experience  that,  however  well  he  may  do  his  work  in  the  fall, 
however  clean  the  surface  may  be  when  winter  sets  in,  the  fol- 
lowing spring,  after  the  frost  is  out  of  the  ground,  will  find  a 
new  crop  in  no  way  distinguishable  from  the  old,  and  which, 
for  all  that  he  can  see,  may  have  rained  down  during  the  win- 
ter's storms.  The  fact  is,  however,  that  they  have  been  actually 
thrown  up,  "heaved  out,"  the  farmers  will  say,  from  below  the 
surface  by  the  frost  which  here  penetrates  not  infrequently  to 
a  depth  of  two  or  more  feet.  As  the  water-soaked  clay  under- 
lying one  of  these  buried  boulders  freezes,  it  expands  upwards, 
since  this  is  the  direction  of  least  resistance.  The  stone  is 
carried  up  bodily  for  a  distance  dependent  on  the  amount  of 
expansion.  When  the  frost  leaves  the  ground,  the  soil  sinks 
back  nearly  to  its  first  position;  but  the  boulder  never  quite 
regains  its  former  place,  being  prevented  by  particles  of  soil, 
or  clay  or  pebbles  which  fall  into  the  cavity  as  the  soil  shrinks 
away  from  it.  The  amount  of  actual  lifting  for  each  season 
may  be  but  slight,  but  as  the  process  goes  on  unceasingly  there 
is  always  an  abundance  of  new  material  at  the  surface  each 
succeeding  spring.  This  heaving  action  of  the  frost  is  abun- 
dantly exemplified  in  these  clay  regions  by  the  throwing  out 
of  fence  posts  and  clover  roots ;  sometimes,  when  the  winter  is 
one  of  frequent  freezing  and  thawing,  causing  the  destruction 
of  a  crop  as  completely  as  though  it  had  been  pulled  up  by  the 
roots.  In  wet  boggy  lands  this  heaving  action  of  frost,  as 
exerted  on  partially  buried  boulders  of  small  size,  is  sometimes 
exemplified  in  a  peculiarly  striking  manner.  The  surface  of 
the  ground  will  be  dotted  here  and  there  with  small  hummocks, 
each  with  a  comparatively  large  crater-like  opening  at  the  top. 
Investigation  reveals  the  fact  that  at  a  distance  of  but  a  few 
inches  at  most  below  the  surface  of  this  crater-like  opening  is 
a  rounded  boulder.  The  heaving  action  of  the  frost  forces  the 
boulder  gradually  upward,  causing  the  turf  to  first  rise  with 


394  THE   REGOLITH 

smooth  rounded  outline,  till,  through  continual  pressure  from 
the  boulder,  it  bursts  at  the  top.  When  the  frost  leaves  the 
ground,  the  boulder  drops  back  a  short  distance,  but  enough 
to  be  quite  out  of  sight,  leaving  the  cavity  at  the  top  filled 
with  mud,  and  looking  —  in  outline  —  like  a  small  mud  volcano. 
So  far  as  the  writer's  observations  go,  the  heaving  action  rarely 
progresses,  in  these  areas,  to  the  point  of  actually  throwing 
the  boulder  out  upon  the  surface.  Each  summer  the  growing 
turf  makes  an  attempt  at  healing  the  wound,  but  each  winter's 
frost  opens  it  once  more,  the  alternating  forces  so  nearly  bal- 
ancing that  little  is  accomplished  after  this  pseudo-volcanic 
stage  is  reached. 

Insects  like  the  boring  bee,  the  burying  beetle,  or  larger  bur- 
rowing animals,  like  the  "  woodchuck  "  of  the  Eastern  states, 
the  prairie  dogs,  badgers,  and  spermophiles  of  the  West,  in  the 
same  way  exert  powerful  though  local  influences  in  admixing 
the  lower  with  the  upper  portions  of  the  soil,  and  through 
allowing  perhaps  a  more  ready  passage  of  water  facilitating 
oxidation  and  decomposition  at  greater  depths.  (Fig.  2, 
PI.  19.) 

While  the  effect  of  these  animals  may  be  comparatively  in- 
conspicuous in  the  regions  east  of  the  Mississippi,  in  the  drier 
regions  of  the  West  the  surface  is  not  infrequently  so  under- 
mined by  burrows  as  to  make  travelling  on  horseback  at  more 
than  a  very  moderate  pace  a  matter  of  grave  difficulty.  W.  P. 
Blake,  in  the  early  reports  of  the  Pacific  Railroad  Survey, 
states  that  the  fine,  silty  soil  of  the  Tulare  valley  in  California 
is  so  undermined  that  it  is  almost  impossible  to  travel  over  it. 
"  Mules  often  break  through  the  thin  crust  and  sink  to  their 
shoulders  in  these  holes." 

The  action  of  plant  life  in  the  accumulation  of  vegetable 
mould  has  been  fully  discussed  under  the  head  of  cumulose 
and  alluvial  deposits.  There  is,  however,  one  phase  of  action 
which  may  well  be  mentioned  here.  A  growing  tree,  as  already 
noted,  sends  its  roots  deep  down  into  the  earth  in  search  of  food 
and  foothold.  So  long  as  the  tree  remains  alive  and  standing, 
in  firm  soil  the  amount  of  change  in  the  soil  itself,  except  in  the 
way  of  abstraction  of  certain.constituents  taken  up  by  the  grow- 
ing plant,  is  presumably  very  small.  When,  however,  the  tree 
dies,  the  roots  slowly  decay,  and  besides  yielding  up  their  con- 
tents to  form  new  soil,  afford  passageway  for  percolating  water 


EFFECT   OF   PLANT  AND   ANIMAL   LIFE 


395 


Forest  Mould. 


FIG.  41. 


with  all  its  attendant  results.  Moreover,  cases  are  by  no  means 
infrequent  in  which  trees  are  upturned  by  the  winds,  bringing 
entangled  in  their  roots  it  may  be  tons  of  soil  and  boulders 
which  in  part  gradually  fall  back  into  the  hole  and  in  part  re- 
main to  form  a  mound 
which  marks  the  spot  long 
after  the  tree  has  de- 
cayed. Into  the  cavity 
thus  formed,  dead  leaves 
and  other  organic  debris 
accumulate,  which  in  time 
form  -deep  rich  loam  to  be 
commingled  with  the  stony 
matter  of  the  soil.  In  sec- 
tions of  the  country  where 
heavy  winds  and  hurri- 
canes are  of  frequent  oc- 
currence, the  efficacy  of 
trees  in  thus  burying  or- 
ganic matter,  and  produc- 
ing a  more  complete  inter- 
mingling of  the  soils,  is  by 
no  means  inconsiderable.1 
The  influence  of  plants  in 
adding  carbon  and  inci- 
dentally carbonic  and  other  FlG>  42 
organic  acids  to  the  soils 

has  been  described  in  previous  pages.  When  plants  die  and 
decay  upon  the  immediate  surface,  there  is  left  only  the  inor- 
ganic matter  or  ash  behind,  the  carbonic  acid  escaping  into  the 
air  or  being  carried  by  rains  into  the  soil.  Hence  it  would 
seem  to  naturally  follow  that  the  soil  where  supporting  an 
abundant  vegetation  should  contain  a  larger  percentage  of 
carbonic  acid  than  the  atmosphere  itself.  That  it  does  not 
contain,  in  all  cases,  a  greater  amount  of  free  carbonic  acid  is 
apparently  brought  out  in  the  table .  from  the  works  of  Bous- 
singault  and  Lewy,  as  quoted  on  p.  178. 

1  Some  of  our  archaeologists  go  so  far  as  to  assert  that  the  stone  implements 
found  buried  several  feet  below  the  surface  in  glacial  deposits,  and  brought  for- 
ward as  proving  the  existence  of  pre-glacial  man,  have  been  brought  into  that 
position  by  just  such  agencies.  See  Holmes,  Early  Man  in  Minnesota,  American 
Geologist,  April,  1893,  p.  228. 


396  THE   REGOLITH 

Bacteria  as  agents  of  nitrification  are  undoubtedly  efficacious 
in  preparing  nitrogeneous  matter  in  the  soils  for  assimilation 
by  growing  plants.  Their  influence  as  decomposers  of  rock 
masses  was  noted  on  p.  203.  According  to  Wiley,1  it  is  highly 
probable  that  organic  nitrogen  in  the  soil,  in  passing  into  the 
form  of  nitric  acid,  exists  at  some  period  of  the  process  in  the 
form  of  ammonia.  The  products  of  nitrification,  he  says,  are 
ammonia,  nitrous  or  nitric  acid,  carbon  dioxide,  and  water. 
The  ammonia  and  nitrous  acid  may  not  appear  in  the  soils  as 
the  final  products  of  nitrification,  as  the  organism  attacks  the 
nitrous  acid  at  once,  converting  it  into  the  nitric  form. 

It  may  at  first  seem  strange  that  man,  who  prides  himself  on 
being  the  highest  type  in  the  animal  kingdom,  as  well  as  the 
only  animal  endowed  with  reasoning  powers,  should  prove  the 
most  destructive ;  yet  such  is  the  case.  Through  prodigality, 
due  in  part  to  thoughtlessness  and  in  part  to  a  wilful  disregard 
for  any  but  immediate  interests,  man  has,  apparently  from  the 
very  beginning  of  his  existence,  so  conducted  himself  with  re- 
lation to  natural  resources  as  to  leave  little  less  than  ruin  in 
his  path.  This  is  true  not  merely  with  reference  to  his  treat- 
ment of  the  soil,  but  of  the  deeper  lying  rocks  and  their  min- 
eral contents.  In  the  name  of  development  he  has  squandered  ; 
through  careless  husbandry  he  has  not  merely  impoverished 
the  soil,  but  in  many  cases  allowed  it  to  run  waste  and  be  lost 
beyond  recovery.  So  long  ago  as  1846,  when  Lyell  made  his 
second  visit  to  America,  he  was  struck  by  the  rapid  denuda- 
tion of  the  land  in  our  Southern  states  due  to  the  reckless  cut- 
ting away  of  the  forests.  He  describes  near  Milledgeville,  in 
Georgia,  a  washout  in  a  lately  deforested  area.  "  Twenty  years 
ago,"  he  writes,  "  before  the  land  was  cleared,  it  [the  washout] 
had  no  existence ;  but  when  the  trees  of  the  forest  were  cut 
down,  cracks  3  feet  deep  were  caused  by  the  sun's  heat  in  the 
clay ;  and  during  the  rains,  a  sudden  rush  of  water  through  the 
principal  crack  deepened  it  at  its  lower  extremity,  from  whence 
the  excavating  power  worked  backwards,  till  in  the  course  of 
20  years,  a  chasm  measuring  no  less  than  55  feet  in  depth,  300 
yards  in  length,  and  varying  in  width  from  20  to  180  feet  was 
the  result.  The  high  road  has  been  several  times  turned  to 
avoid  this  cavity,  the  enlargement  of  which  is  still  proceeding, 
and  the  old  line  of  road  may  be  seen  to  have  held  its  course, 
1  Principles  and  Practice  of  Agricultural  Analysis,  p.  464. 


EFFECT   OF   PLANT   AND   ANIMAL   LIFE  397 

directly  over  what  is  now  the  widest  part  of  the  ravine.  In 
the  perpendicular  walls  of  this  great  chasm  appear  beds  of  clay 
and  sand,  red,  white,  yellow,  and  green,  produced  by  the  de- 
composition in  situ  of  hornblendic  gneiss,  with  layers  of  veins 
of  quartz,  which  remain  entire,  to  prove  that  the  whole  mass 
was  once  solid  and  crystalline."1 

The  same  lack  of  foresight  or  wanton  disregard  for  coming 
generations  is  still  manifested,  and  every  muddy  stream  bears 
downward  to  the  sea  an  increased  load  of  silt  from  lands  im- 
properly cultivated  and  from  which  every  rain  removes  a  por- 
tion of  the  finest  and  richest  of  the  soil,  leaving  behind  but  the 
barren  gravel,  channelled  it  may  be  beyond  the  possibility  of 
cultivation.  McGee2  has  more  recently  made  observations  of 
a  similar  nature  in  southern  Mississippi,  where  the  softer  loam 
of  the  Columbia  formation,  which  here  forms  the  soil,  has 
been  allowed  to  become  eroded  down  to  the  barren  sandy  loam 
of  the  Lafayette.  "  Old  fields  are  denuded  by  the  acre,  leaving 
mazes  of  pinnacles  divided  by  a  complex  network  of  runnels 
glaring  red  toward  the  sun  and  sky  in  strong  contrast  to  the 
rich  verdure  of  the  hillsides  never  deforested ;  the  plantations, 
mansions,  and  '  quarters '  are  undermined,  and  whole  villages, 
once  the  home  of  wealth  and  luxury,  are  being  swept  away  at 
the  rate  of  acres  for  each  year/' 

"  The  ravages  committed  by  man,"  writes  Marsh,3  "  subvert 
the  relations  and  destroy  the  balance  which  nature  had  estab- 
lished between  her  organized  and  her  inorganic  creations,  and 
she  avenges  herself  upon  the  intruder  by  letting  loose  upon  her 
defaced  provinces  destructive  energies  hitherto  kept  in  check 
by  organic  forces  destined  to  be  his  best  auxiliaries,  but  which 
he  has  unwisely  dispersed  and  driven  from  the  field  of  action. 
When  the  forest  is  gone,  the  great  reservoir  of  moisture  stored 
up  in  its  vegetable  mould  is  evaporated,  and  returns  only  in 
deluges  of  rain  to  wash  awa}r  the  parched  dust  into  which  that 
mould  has  been  converted.  The  well-wooded  and  humid  hills 
are  turned  to  ridges  of  dry  rock,  which  encumbers  the  low 
grounds  and  chokes  the  watercourses  with  its  debris,  and  — 
except  in  countries  favored  with  an  equable  distribution  of  rain 

1  Lyell,  Principles  of  Geology,  9th  ed.,  1846,  p.  204. 

2  12th  Ann.  Rep.  U.  S.  Geol.  Survey,  1890-91. 

3  The  Earth  as  modified  by  Human  Action,  by  Geo.  P.  Marsh,  a  new  edition 
of  Man  and  Nature,  pp.  43, 44. 


398  THE   REGOLITH 

through  the  seasons,  and  a  moderate  and  regular  inclination  of 
surface — the  whole  earth,  unless  rescued  by  human  art  from  the 
physical  degradation  to  which  it  tends,  becomes  an  assemblage 
of  bald  mountains,  of  barren,  turfless  hills,  and  of  swampy  and 
malarious  plains.  There  are  parts  of  Asia  Minor,  of  northern 
Africa,  of  Greece,  and  even  of  Alpine  Europe,  where  the  opera- 
tion of  causes  set  in  action  by  man  has  brought  the  face  of  the 
earth  to  a  desolation  almost  as  complete  as  that  of  the  moon ; 
and  though,  within  that  brief  space  of  time  which  we  call  '  the 
historical  period,'  they  are  known  to  have  been  covered  with 
luxuriant  woods,  verdant  pastures,  and  fertile  meadows,  they 
are  now  too  far  deteriorated  to  be  reclaimable  by  man,  nor  can 
they  become  again  fitted  for  human  use,  except  through  great 
geological  changes,  or  other  mysterious  influences  or  agencies 
of  which  we  have  no  present  knowledge,  and  over  which  we 
have  no  prospective  control.  The  earth  is  fast  becoming  an 
unfit  home  for  its  noblest  inhabitant,  and  another  era  of  equal 
human  crime  and  human  improvidence,  and  of  like  duration 
with  that  through  which  traces  of  that  crime  and  that  improvi- 
dence extend,  would  reduce  it  to  such  a  condition  of  impover- 
ished productiveness,  of  shattered  surface,  of  climatic  excess, 
as  to  threaten  the  depravation,  barbarism,  and  perhaps  even 
extinction  of  the  species." 


LIST  OF  AUTHORS  CITED  OR  REFERRED  TO 


Agassiz,  L.,  175. 

Alexander,  H.  F.,  242. 

Aughey,  S.,  331. 

Bartlett,  W.  H.,  180. 

Bayley,  W.  S.,  7f>,  78,  80. 

Beaumont,  Elie  de,  160. 

Becker,  G.  F.,  235,  301. 

Bell,  Robert,  194,  243,  246,  275. 

Belt,  T.,  175,  261,  277,  280,  298. 

Berthier,  P.,  237. 

Beyer,  M.,  178. 

Bischof,  G.,  20,  27,  191, 192. 

Bkias,  J.,  102. 

Blake,  W.  P.,  121,  126, 185,  247,  250,  34(5, 

348,  394. 

Bolton,  H.  Carrington,  202. 
Bonney,  T.  G.,  246. 
Boussingault,  J.  B.,  176. 
Branner,  J.  C.,  Ill,  175, 179,  188,  203,  278. 
Brogger,  W.  C.,  64. 
Brougniart,  A.,  87,  175,  237. 
Brown,  A.  P.,  29. 
Bruner,  H.  L.,  344. 
Buchanan,  J.  V.,  204. 
Caldcleugh,  Alexander,  193. 
Chamberlain,  T.  C.,  278,  301,  303. 
Cholfat,  P.,  255. 
Clark,  W.  B.,  134. 
Clark,  W.  C.,  118. 
Cloez,  176. 
Collier,  P.,  369. 
Cointe  de  la  Hure,  188. 
Crosby,  W.  O.,  138,  189,  255,  353,  385. 
Cross,  C.W.,  35,  62,  71,81. 
Culver,  G.  E.,  279. 
Gushing,  H.  P.,  279. 
Dana,  E.  S.,  31,  127. 
Dana,  Professor  J.  D.,  49,  57,  117,  198, 

235,  251,  253,  262. 
Darton,  N.  L.,  312. 
Darwin,  E.,  175,  233,  292,  392. 
Daubree,  A.,  16,  197,  376. 
Davis,  W.  M.,  18(5. 
Davidson,  C.,  287. 
Dawson,  J.  W.,  291, "334. 


De  Luca,  176. 

Derby,  O.  A.,  188,  277. 

Diller,  J.  S.,  87,  92. 

Dutton,  C.  E.,  196. 

Dwight,  297. 

Dyer,  B.,  202. 

Ebelmen,  M.,  237. 

Egleston,  Thomas,  184. 

Ewing,  A.  L.,  194. 

Failyer,  G.  H.,  176. 

Fernow,  B.  E.,  282. 

Fesca,  Dr.  Max,  243. 

Forbes,  184. 

Forschammer,  J.  G.,  237. 

Fournet,  175,  237. 

Fulton,  R.  L.,  280. 

Furlouge,  W.  H.,  277. 

Geikie,  A.,  2,  146,  201,  288. 

Geikie,  James,  357. 

Geldmacher,  Max,  236. 

Gesner,  H.  S.,  317. 

Gilbert,  G.  K.,  50,  185,  256,  349. 

Gordon,  C.  H.,  108. 

Griswold,  L.  S.,  111.  , 

Gumbel,  C.  W.,  28,  88. 

Hall,  C.  W.,  and  Sardeson,  F.  W.,  161,  250. 

Harker,  A.,  39. 

Hartt,  C.  F.,  175,  280. 

Hawes,  G.  W.,  46,  75,  87, 170. 

Ha  worth,  E.,  25. 

Hayden,  F.  V.,  252. 

Hayes,  C.  W.,  109,  194. 

Heusser  and  Claraz,  175,  228,  251. 

Hilgard,  E.  W.,  333,  346,  300, 367,  369, 371, 

374. 

Hitchcock,  C.  H.,  68. 
Hitterman,  239. 
Hobbs,  W.  H.,  218. 
Holmes,  W.  H.,  395. 
Hovey,  E.  O.,  230. 
Hunt,  T.  S.,  86,  99,  124,  159,  258. 
Iddings,  J.  P.,  22,  39,  57,  60,  64,  71,  72,  81. 
Irving,  R.  D.,  278. 
Johnson,  S.  W.,  177,  178. 
Johnson  and  Blake,  136. 


400 


LIST  OF  AUTHORS   CITED   OR   REFERRED  TO 


Johnstone,  Alexander,  189. 

Jones,  T.  Rupert,  317. 

Judd,  J.  W.,  284,  321. 

Julien,  A.  A.,  190. 

Kalkowski,  E.,  75. 

Kemp,  J.  F.,  81,  86,  87,  171. 

Kerr,  W.  C.,  286. 

Keyes,  C.  R.,  25. 

Kidder,  J.  H.,  179. 

King,  F.  H.,  381. 

King,  Clarence,  71. 

Klement,  M.  C.,  160. 

Kletzinsky,  W.,  176. 

Kuhn,  M.  Levy,  89. 

Layard,  A.  H.,  293. 

Le  Conte,  J.,  258. 

Lemberg,  J.,  18,  217,  374. 

Lindgren,  W.,  75,  274. 

Livingstone,  David,  183. 

Loftus,  293. 

Loughbridge,  R.  H.,  365,  366. 

Lyell,  Sir  Charles,  396. 

Marsh,  George  P.,  183,  297. 

McGee,  W.  J.,  301,  312,  323,  397. 

Meister,  381. 

Merrill,  G.  P.,  37,  47,  54,  81,  87, 

115,  154,  159,  206,  218,  349. 
Mills,  J.  E.,  175,  203,  273. 
Muller,  Alex,  371. 
Miiller,  R.,  192. 
Munroe,  C.  E.,  190. 
Miintz,  A.,  203. 
Miintz  and  Aubin,  179. 
Miintz  and  Maracano,  373. 
Murakozky,  K.  V.,  238. 
Neumayer,  M.,  302. 
Newberry,  J.  S.,  118,  356. 
Nordenskiold,  A.  E.,  242. 
Oldham,  R.  D.,  311. 
Orton,  Edward,  117, 118, 124. 
Owen,  D.  D.,  111. 
Packard,  R.  L.,  108,  376. 
Pallarsen,  89. 
Penrose,  R.  A.  F.,  231,  232. 
Pirsson,  L.  V.,  64. 
Pliny,  73,  90. 
Porter,  J.  B.,  266. 
Potter,  W.  B.,  265,  275,  276. 
Prestwich,  Joseph,  65,  260. 
Prichard,  30. 
Pumpelly,  R.,  275,  277. 
Purrington,  C.  W.,  279. 
Read,  T.  Mellard,  194. 
Redwood,  Boverton,  129. 
Retgers,  J.  W.,  349. 
Reusch,  H.,  250. 


113, 


Richthofen,  F.  von,  63,  85. 

Rogers  Brothers,  191. 

Rohrbach,  C.  E.  M.,  89. 

Roscoe  and  Schorlemmer,  4. 

Rose,  G.,  79,  89. 

Roseubusch,  H.,  57,  62,  70,  72,  74,  82,  93, 

97,  98. 
Rosiere,  73. 
Roth,  Justus,  72,  94,  101,  103,  208,  239, 

256. 
Russell,  I.  C.,  112,  201,  266,  279,  280,  284, 

296,  301,  333,  385. 
Rutley,  F.,  Ill,  194. 
Safford,  J.  M.,  267. 

Salisbury,  R.  D.,  278,  287,  301,  303,  352. 
Schlosing,  203. 
Schutze,  R.,  228. 

Shaler,  N.  S.,  181,  197,  318,  336,  389,  498. 
Smith,  Angus,  179. 
Sorby,  H.  C.,  26,  38,  199,  243,  342. 
Spurr,  J.  E.,  107. 
Stanley,  H.  M.,  183. 
Stejneger,  L.,  199. 
Stone,  G.  H.,  186. 
Storer,  F.  H.,  191,  202. 
Strabo,  90. 

Streeruwitz,  H.  von,  182. 
Streng,  A.,  86. 
Teall,  J.  J.  H.,  24,  74,  90. 
Theiiard,  P.,  190. 
Thompson,  Wyville,  247. 
Tornebohm,  A.  E.,  87,  89. 
Tschermak,  G.,  24. 
Van  Den  Broeck,  E.,  178,  258. 
VanHise.C.  R.,  106.     . 
Vom  Rath,  G.,  255. 
Von  Buch,  L.,  83. 

Wadsworth,  Dr.  M.  E.,  57,  68,  85,  97, 254. 
Weed,  W.  H.,  109. 
Werner,  A.  G.,  73. 
Whitaker,  W.,  267. 
Whitney,  J.  D.,  68,  127,  278,  328. 
Whitney,  Milton,  287,  307-309,  313,  340, 

344,  353,  379. 
Wichman,  A.,  170. 
Widogradsky,  203. 
Wiley,  H.  W.,  178,  316. 
Williams,  G.  H.,  63,  72,  86,  96,  99,  100, 

156,  216. 

Williams,  J.  F.,  64. 
Willis,  Bailey,  52. 
Winchell,  N.  H.,  297. 
Wolff,  Professor  J.  E.,  93. 
Woodward,  J.  B.,  186. 
Wurtz,  H.,  127. 
Zirkel,  F.,  38,  57,  68,  80,  87,  89. 


INDEX 


Abrasive  action  of  wind-blown  sand,  185. 

Acid  rocks,  meaning  of  term,  64. 

Acmite,  22. 

Adobe,  139,  332. 

JEgerine,  22. 

./Eolian  deposits,  344. 

^Eolian  rocks,  153 ;  defined,  58. 

Agalmatolite,  116. 

Age  of  soils,  386. 

Air  in  motion,  effects  of,  189. 

Alabaster,  117. 

Alaska,  rock-weathering  in,  279,  284. 

Albertite  described,  127. 

Albite  as  a  rock  constituent,  16. 

Alkalies  in  soils,  371. 

Alkaline  carbonates,  when  formed,  372; 
in  soils,  371 ;  formed  during  weather- 
ing, 205. 

Alkaline  silicates  in  soils,  370. 

Allanite,  25. 

Allotriomorphic  minei-als  defined,  41. 

Alluvial  cones  defined,  54. 

Alluvial  deposits,  320. 

Alteration  defined,  174. 

Alum  shale,  138. 

Aluminum  as  a  constituent  of  the  earth's 
crust,  5. 

Amber,  128. 

Amianthus,  115. 

Ammonia  in  atmosphere,  177. 

Ammonium  sulphate,  influence  in  decom- 
posing feldspars,  178. 

Amorphous,  definition  of,  40. 

Ainphiboles  as  rock  constituents,  19. 

Amygdaloidal  structure,  34. 

Anacostia,  deposits  of  the,  323. 

Analyses,  calculations  of,  210;  discus- 
sion of,  212. 

Anamesite,  92. 

Andesites,  83. 

Andesitic  rocks,  induration  of  surface, 
2.55. 

Anhydrite  described,  118. 

Animal  life,  effect  on  soils,  389. 

Anorthite  as  a  rock  constituent,  17. 


Anorthit-gesteine,  89. 

Anthracite  coal,  150. 

Antique  porphyry,  83. 

Ants,  effect  on  soils,  389 ;  as  promoters  of 
rock  decomposition,  204. 

Apatite  as  a  rock  constituent,  27. 

Apo-rhyolite,  72. 

Appalachian  Mountain  system,  material 
eroded  from,  196. 

Appomattox  formation,  312. 

Aqueo-glacial  clays,  334. 

Aqueous  rocks,  105 ;  defined,  58. 

Aragonite  as  a  rock  constituent,  26. 

Arenaceous  group,  the,  131. 

Argillaceous  rocks  described,  135. 

Argillites,  137;  fissile,  170;  Harford 
County,  Maryland,  weathering  of,  229. 

Arkansas  River  referred  to,  289. 

Asphaltum  described,  125. 

Atmosphere,  action  of,  176. 

Augite,  molecular  alteration  of,  39;  rela- 
tive durability  of,  235. 

Augite  porphyrite,  90;  Montana,  disin- 
tegration of,  235. 

Augite  vitrophyrite,  90. 

Augitite  described,  101. 

Auriferous  sands,  origin  of,  266. 

Bacteria,  as  agents  of  nitrification,  396; 
decomposing  action  of,  203. 

Banding  in  gneisses,  origin  of,  165. 

Barbadoes  Island,  volcanic  dust  on,  298. 

Barite  described,  118. 

Barium  as  a  constituent  of  the  earth's 
crust,  7. 

Basalt,  described,  90;  Bohemia,  weather- 
ing of,  223;  Haute  Loire,  France, 
weathering  of,  223. 

Basalts,  geographical  distribution  in 
United  States,  92 ;  weathering  of,  262. 

Basanite  described,  94. 

Base,  definition  of,  40. 

Basic  rocks,  meaning  of  term,  64. 

Beach  sands,  341. 

Beauxite  described,  108. 


401 


402 


INDEX 


Bedded  rocks  defined,  53. 

Bedded  structure,  34. 

Bermuda,  weathering  of  limestones  in, 

247. 

Binary  granite,  68. 
Biotite  as  a  rock  constituent,  23. 
Bitumen,  125. 
Bituminous  coals,  149. 
Bituminous  dolomite  of  Chicago,  145. 
Black  earth,  Russian,  318. 
Bleaching  of  rocks  on  exposure,  257. 
Bluegrass  soil,  382. 
Bog  of  Allen,  317. 
Bogs,  classification  of,  317. 
Boss,  defined,  50. 

Boss-like  form  accentuated  by  joints,  245. 
Botryoidal  structure,  37. 
Boulder  clay,  138. 
Boulder  clays,  353. 
Boulders,  of   decomposition  resembling 

those  of    the   drift,  242;    formed    by 

weathering,  244. 
Bowenite,  116. 
Breccia,  133. 

Brecciated  limestones,  139. 
Brecciated  structure,  38. 
Bronzitite,  100. 
Brown  hematite,  29,  107. 
Brownstone,  133. 

Cabook,  formation  of,  242. 

Calc  sinter,  112. 

Calcareous  group  of  rocks,  137. 

Calcareous  rocks,  143;  rate  of  weather- 
ing, 272. 

Calcite  as  a  rock  constituent,  25. 

Calcium  as  a  constituent  of  the  earth's 
crust,  6. 

Calcium  carbonate,  amount  annually  re- 
moved in  solution,  194. 

Camptonite,  8. 

Cannel  coal,  150. 

Cape  Cod,  wind  action  on,  297. 

Carbonates  of  alkalies,  influence  of,  238. 

Carbonates  of  the  alkalies  formed  during 
weathering,  205. 

Carbonates,  production  of,  during  weath- 
ering, 205. 

Carbonaceous  rocks,  148. 

Carbonic  acid,  influence  of,  in  feldspathic 
decomposition,  237,  239;  amount  annu- 
ally brought  to  the  surface,  179 ;  in  air 
of  soils,  178;  in  the  atmosphere,  178. 

Carboniferous  soils,  386. 

Catlinite,  139. 

Cavernous  structure,  38. 

Cellular  structure,  38. 

Ceylon,  rock  disintegration  in,  242. 

Chalcedony,  110. 


Chalk,  143 ;  decomposition  of,  267. 

Chemical  composition  of  rocks,  44. 

Chemical  elements  constituting  rocks,  4. 

Chert,  110 ;  of  Arkansas,  weathering  of, 
231 ;  of  Missouri,  weathering  of,  230. 

Chilian  nitrates,  origin  of,  373. 

Chlorides,  119. 

Chlorite  as  a  rock  constituent,  30. 

Chrysotile,  115. 

Citric  acid,  solvent  property  of,  202. 

Classification  of  soils,  381. 

Clastic  rocks,  129 ;  classification  of,  130. 

Clastic  structure,  34. 

Clay  concretions,  formation  of,  37. 

Clay,  defined,  135;  effect  on  soils,  368; 
protective  influence  of,  254. 

Clay  ironstone,  114. 

Clay  slates,  137. 

Clays,  aqueo-glacial,  334. 

Climate,  influence  of,  on  weathering,  278. 

Clinton  iron  ores,  origin  of,  266. 

Coefficient  of  cubical  expansion  of  min- 
erals, 268. 

Coking  coals,  150. 

Cold,  effect  on  rocks,  180. 

Colloidal  structure,  33. 

Colluvial  deposits,  319. 

Color ;  changes  incidental  to  weathering, 
257 ;  of  rocks,  45 ;  of  soils,  384  ;  of  soils, 
cause  of,  385 ;  variation,  cause  of,  47. 

Colorado  Kiver,  erosion  by,  196. 

Columbian  formation,  312. 

Columnar  structure,  38. 

Complexity  of  structure  favoring  disinte- 
gration, 250. 

Concentric  exfoliation,  244;  not  indica- 
tive of  an  original  concretionary  struct- 
ure, 245. 

Concentric  structure  inevitable  to  joint- 
ing, 245. 

Concretionary  structure,  35;  in  granite, 
246;  in  crystalline  rocks,  37. 

Conductivity  of  rocks,  unequal,  184. 

Conglomerate,  133. 

Conservative  action  of  plants,  202. 

Contact  metamorphism,  157. 

Contours  incidental  to  weathering,  259. 

Coprolite  nodules,  152. 

Coquina,  143. 

Coral  limestone,  143. 

Corroded  surfaces,  irregularity  of,  250. 

Corsica,  weathering  of  granite  on,  250. 

Crayfish,  effects  on  soils,  391. 

Creeping  of  soil  cap,  287. 

Crenic  acid,  190. 

Crystalline  limestones  and  dolomkes, 
162. 

Crystalline  schists,  the,  168. 

Crystalline  structure,  33. 


INDEX 


403 


Crystallites  defined,  41. 
Cumulose  deposits,  313. 

Dacite,  84. 

Daubree's  experiments  in  rock  tritura- 
tion,  17,  197. 

Decay,  time  limjt  of,  272;  of  rocks,  how 
characterized,  212. 

Decomposition  and  disintegration,  dis- 
crimination between,  283. 

Decomposition,  depth  of,  278 ;  following 
disintegration,  243;  incident  to  ero- 
sion, 197;  of  fragmental  rocks,  228;  of 
greenstone  dikes,  effects  of,  244 ;  of 
rocks,  chemical  processes  involved  in, 
238 ;  of  shells  through  the  aid  of  salt, 
233 ;  natural  acceleration  of,  205. 

Degeneration  of  rocks,  174. 

Degradation  of  North  America, rate  of  ,196. 

Delta  deposits,  320. 

Delta  of  the  Nile,  section  of,  321. 

Deoxidation,  187 ;  by  marine  animals, 204. 

Desert  varnish,  256. 

Devil's  Tower,  origin  of,  261. 

Deweylite,  116. 

Diabase,  described,  87;  mandelstein,  90; 
Medford,  Massachusetts,  weathering 
of,  218 ;  porphyrite,  90 ;  Venezuela, 
weathering  of,  222. 

Diallogite,  100. 

Diamonds,  origin  of,  98. 

Diatom aceous  earth,  141. 

Dichte  diabase,  90. 

Dike  denned,  50. 

Diluvium,  rouge  et  gres,  258. 

Diorite,  Albemarle  County,  Virginia, 
weathering  of,  224. 

Diorite-andesite  group,  81. 

Diorites,  87. 

Discoloration,  above  drainage  level,  258; 
incidental  to  weathering,  257. 

Discussion  of  analyses,  234. 

Disintegration  of  rocks  in  Lower  Califor- 
nia, 183 ;  prevented  by  surroundings, 
252;  without  decomposition,  241. 

District  of  Columbia,  rock-weathering 
in,  283. 

Ditroite,  79. 

Dolerite,  92. 

Dolomite,  as  a  rock  constituent,  26 ;  de- 
scribed, 145 ;  origin  of  name,  163 ;  ori- 
gin of,  by  metasomatosis,  159. 

Dolomites,  162. 

Dolomitic  limestones,  disintegration  of, 
250  ;  weathering  of,  239. 

Drift,  extent  of,  291. 

Drumlins,  355;  defined,  55. 

Dune  defined,  55. 

Dune  sand,  chemical  composition,  350. 


Dunite,  97. 

Dust,  in  rain  and  snow  falls,  344;  vol- 
canic, 298,  349. 

Dust  soils,  345. 

Dust  storms,  292 :  in  Dakota,  293 ;  in  Mon- 
tana and  Nevada,  294. 

Dynamic  metamorphism,  156. 

Earth's  crust,  thickness  of,  2. 

Earthworms,  effects  on  soils,  392. 

Eclogite,  170. 

Effacement  of  characteristics  by  weath- 
ering, 262. 

Effusive  .rocks,  characteristics  of,  61 ; 
defined,  60. 

Elaeolite  as  a  rock  constituent,  18. 

Elreolite  syenites,  78. 

Elaeolite  syenite  porphyry,  79. 

Elaterite  described,  126. 

Elvanite,  70. 

Eozoon  Canadense,  159,  163;  origin  of, 
116. 

Epidiorite,  89. 

Epidote,  as  a  rock  constituent,  25 ;  altera- 
tion of,  25. 

Erosion  by  rivers,  196. 

Eruptive  rocks,  59. 

Eskers,  290,  356. 

Eucrite,  89. 

Eulysite,  97. 

Eurite,  70. 

Exfoliated  rocks,  shape  and  size  of  flakes, 
182. 

Exfoliation,  attended  by  gun-like  reports, 
182  :  due  to  heat  and  cold,  181 ;  of  rocks 
on  Cape  Cod,  182. 

Expansion  through  hydration,  188. 

Extent  of  weathering,  276 :  in  Brazil, 
Colorado,  District  of  Columbia,  Mis- 
souri, Nicaragua,  South  Africa,  South 
America,  277. 

Fault  defined,  53. 

Feldspars,  as  rock  constituents,  13;  de- 
composition of,  17. 

Feldspathic  decomposition,  process  of, 
237;  in  Comstock  Lode,  235;  by  fresh 
water,  238 ;  influenced  by  ammonium 
sulphate  and  sodium  chloride,  178. 

Feldspar  porphyry  of  Iron  Mountain, 
weathering  of,  265. 

Feldspars,  relative  durability  of,  235. 

Felsitic  structure,  33. 

Felsite  pitchstone,  70. 

Felsophyr,  70 

Felstone,  70. 

Ferrous  carbonate,  solubility  of,  239. 

Fertility  of  soil  dependent  on  physical 
condition,  379. 


404 


INDEX 


Fichtellite,  129. 

Fiorite,  109. 

Fissile  argillites,  roofing  slates,  170. 

Flagstone,  133. 

Flexible  sandstone,  134. 

Flint,  110. 

Flood  plain  of  the  Mississippi,  323. 

Fluidal  or  fluxion  structure,  34. 

Fogs,  indices  of  dust  in  atmosphere, 
344. 

Foliated  or  schistose  rocks,  164. 

Foliated  structure,  34. 

Forellenstein,  87. 

Forests,  buried  by  sand,  295;  influence 
of,  280 ;  protective  action  of,  282. 

Fourchite,  79. 

Foyaite,  79. 

Foyaite-phonolite  group,  77. 

Fracture  of  rocks,  48. 

Fragmental  structure,  34. 

Freestone,  133. 

Freezing  water,  disintegrating  action  of, 
198. 

Frontal  aprons,  356. 

Frontal  moraines,  355. 

Frost,  action  in  accelerating  decomposi- 
tion, 278 ;  action  on  soil,  367 ;  disin- 
tegrating action,  199;  heaving  effects 
on  boulders,  393;  supposed  protective 
action  of,  278. 

Gabbro  described,  85. 

Gabbro-basalt  group,  85. 

Garnet  rock,  170. 

Garnserite,  formation  of,  226. 

Garnetite,  170. 

Geest,  301. 

Gem  sands  of  Ceylon,  origin  of,  266. 

Genetic  relationship  of  rocks,  64. 

Geological  age,  of  soils,  389 ;  a  basis  for 
classification,  63. 

Geyserite,  109. 

Gilsonite,  127. 

Glacial  deposits,  351. 

Glacial  detritus,  amount  of,  201. 

Glacial  drift,  extent  of,  291. 

Glacial  lakes,  extinction  of,  289;  filling 
of,  326. 

Glacial  landscape,  291. 

Glacial  moraine,  290. 

Glacial  soil  of  Cape  Elizabeth,  composi- 
tion of,  364. 

Glacier,  the,  as  an  erosive  agent,  200. 

Glaciers,  as  agents  of  transportation,  200 
289. 

Glass  abraded  by  wind-blown  sand,  185. 

Glauconite,  31,  134. 

Glauconitic  marl,  134. 

Globulitos  defined,  41. 


Gneiss,  Albemarle  County,  Virginia,  de- 
generation of,  213. 

Gneisses,  the,  164. 

Grahamite  described,  127. 

Granite,  described,  65 ;  extent  of  weather- 
ing in  District  of  Columbia,  276. 

Granitell,  68. 

Granite-liparite  group,  65. 

Granite  porphyry,  68. 

Granite  soil  defined,  383. 

Grauitite,  67. 

Granofelsophyr,  70. 

Granophyr,  70. 

Granular  structure,  34. 

Granulite,  167. 

Grauwacke,  133. 

Graphic  granite,  67. 

Gravels  superficially  oxidized,  258. 

Gravity,  action  of,  in  transporting  debris, 
286. 

Greenland,  rock-weathering  in,  278. 

Greensand  marl,  134. 

Greenstone,  81. 

Greisen,  68. 

Greywacke,  133. 

Ground-mass  defined,  40. 

Ground  moraine,  352. 

Gruss,  301. 

Guano,  151. 

Gypsum  described,  117. 

Halleflinta,  167. 

Hardpan,  368. 

Harzburgite,  97. 

Hatchettite,  129. 

Hatteras  and  Henlopen,  sand  dunes  of, 
295. 

Heat,  action  on  pebbles  in  Arabia  Petrea, 
183 ;  expansive  action  on  rocks,  180. 

Heat  and  cold,  as  agents  of  decomposi- 
tion, 180  ;  effects  of,  in  Africa,  183  ; 
effects  limited  to  surface,  183;  most 
effective  on  slopes,  184. 

Heavy  spar,  118. 

Hematite,  106 ;  as  a  rock  constituent,  28. 

Holocrystalline,  definition  of,  40. 

Hornblende,  as  a  rock  constituent,  19; 
decomposition  of,  20;  relative  durabil- 
ity of,  235. 

Hornblende  picrite,  97. 

Hornblendite,  100. 

Humic  acid,  189. 

Humidity,  weathering  influenced  by,  270. 

Hyaline  andesite,  85. 

Hyalite  formed  during  feldspathic  de- 
composition, 238. 

Hyalobasalt,  92. 

Hyaloliparite,  72. 

Hyalomelan,  92. 


INDEX 


405 


Hyalotrachyte,  77. 

Hydration,  187  ;  importance  of,  188,  234, 

253,  278 ;  of  micas,  189. 
Hydraulic  limestone,  145. 
Hydrocarbou  compounds,  description  of, 

121. 

Hydro-metamorphism,  101. 
Hyperite,  87. 
Hypersthenite,  100. 
Hypocrenic  acid,  190. 
Hypocrystalliiie,  definition  of,  40, 

Ice,  disintegrating  action  of,  198 ;  influ- 
ence in  transporting  rock  debris,  287 ; 
mechanical  action  of,  195. 

Idiomorphic  minerals  defined,  41. 

Igneous  rocks,  59;  defined,  57. 

Ilmenite  as  a  rock  constituent,  28. 

Induration,  cause  of,  255;  of  rocks  on 
exposure,  254 ;  of  sandstone  by  igneous 
contacts,  261. 

Infusorial  earth,  141. 

Insects,  effects  on  soils,  394. 

Intrusive  rocks  defined,  GO. 

Inundated  lands,  classification  of,  318. 

Iron,  as  a  constituent  of  the  earth's  crust, 
5 ;  removed  in  form  of  ferrous  sul- 
phate, 239;  removed  in  form  of  pro- 
toxide carbonate,  239;  variation  in 
solubility,  239. 

Iron  Mountain,  Missouri,  pre-Silurian 
weathering  of,  276. 

Iron  ores  as  rock  constituents,  27. 

Iron  pyrites  as  a  rock  constituent,  29. 

Itacolumite,  133. 

Itacolumites,  Brazilian,  weathering  of, 
228. 

Jasper,  110. 

Joints,  as  aids  to  weathering,  244 ;  cause 
of,  245 ;  influence  of,  in  producing 
boulders,  244;  influence  in  producing 
boss-like  forms,  245. 

Kalk  diabase,  90. 

Kames,  290. 

Kaolin,   116,   136,  205;    composition  of, 

309;  origin  of,  308. 
Kaolinite  distinct  from  kaolin,  309. 
Kaolinization  defined,  18. 
Keratophyr,  76. 
Kersantite,  82. 
Kimberlite,  98. 
Kinds  of  rocks,  50. 
Kinzigkite,  170. 
Konlite,  129. 

Krakatoa,  dust  from,  298. 
Ktaadn  Iron  AVorks  referred  to,  107. 
Kugel  porphyry,  70. 


Labradorite  as  a  rock  constituent,  17. 

Laccolite  defined,  50. 

Lake  Agassiz,  deposits  in,  290. 

Lake  Asphaltites,  120. 

Lakes,  filling  of,  314 ;  transient  charac- 
ter of,  326. 

Laminated  or  banded  structure,  38. 

Landscape,  glacial,  291. 

Lapilli,  140. 

Laterite,  139,  310. 

Laurvikite,  79. 

Lava  defined,  51. 

Leda  clays,  334. 

Leopardite,  70. 

Leptinite,  107. 

Leucite  as  a  rock  constituent,  18. 

Leucite  basalt,  103. 

Leucite-nepheline  rocks,  102. 

Leucite  rocks  described,  102. 

Leucitite,  103. 

Leucitophyr,  80. 

Leucophyr,  88. 

Leucoxene,  28. 

Lherzolite,  97. 

Lichens,  action  of,  201. 

Liebuerite,  79. 

Lignite,  149. 

Limburgite  described,  98. 

Lime  carbonate,  decomposing  action  of, 
370. 

Lime  in  soils,  366. 

Limestone,  unequal  weathering  of,  250; 
weathering  of,  232. 

Limestone  residuals,  character  of,  303. 

Limestone  soils  poor  in  lime,  259. 

Limestones,  143;  and  dolomites,  162; 
corroded  by  acids,  194;  corroded  by 
meteoric  waters,  259;  unequal  indura- 
tion of,  247;  variation  in  composition, 
147. 

Limit  of  diminution  in  size  of  particles 
by  erosion,  197. 

Limonite,  107  ;  as  a  rock  constituent,  29. 

Liparite  described,  70. 

Litchfieldite,  79. 

Lithophysfu,  72. 

Loess,  139,  290,  327. 

Logans,  or  tors,  252. 

Lower  California,  rock-weathering  in, 
283. 

Lumachelle,  143. 

Lustre,  48. 

Luxullianite,  70. 

Lydian  stone,  111. 

Magma,  definition  of,  59. 
Magnesian  limestones,  145. 
Magnesia    removed    in  excess  of   lime, 
239. 


406 


INDEX 


Magnesium  as  a  constituent  of  the  earth's 
crust,  6. 

Magnesite,  113. 

Magnetite  as  a  rock  constituent,  27. 

Man,  has  squandered  in  the  name  of 
development,  397;  ravages  committed 
by,  397. 

Marbles,  163. 

Marcasite  as  a  rock  constituent,  29. 

Marginal  moraines,  355. 

Marine  animals,  influence  of,  on  marine 
muds,  204. 

Marl,  146. 

Marmolite,  116. 

Marsh  gas,  121. 

Marsh  lands,  reclaimable  areas,  340. 

Martite,  106. 

Massive  structure,  34. 

Material  lost  through  weathering,  208. 

Materials  lost  during  decomposition,  pro- 
portional amounts,  234. 

Mechanical  action  of  water  and  ice,  195. 

Mechanical  disintegration  most  active  in 
regions  of  extreme  temperatures,  182. 

Melaphyr  described,  90. 

Melaphyrs  and  augite  porphyrites,  90. 

Melilite  basalt,  92. 

Menaccanite  as  a  rock  constituent,  28. 

Metamorphic  rocks,  155;  defined,  58. 

Metamorphism  defined,  155. 

Metasomatosis  defined,  158. 

Miascite,  79. 

Mica,  relative  durability  of,  236. 

Micaceous  sandstone,  cause  of  weather- 
ing, 189. 

Micas,  alteration  and  decomposition  of, 
23 ;  as  rock  constituents,  22. 

Microcline  as  a  rock  constituent,  16. 

Microcrystalline  structure,  variation  in, 
41. 

Micro-granite,  70. 

Microlites  defined,  40. 

Microlitic  structure,  33. 

Micropegmatite,  70. 

Microscope  used  in  geology,  38. 

Microscopic  structure,  38  ;  of  rocks,  33. 

Microscopic  study  of  rocks,  efficiency  of, 
39. 

Mineral  caouchouc,  126. 

Mineral  composition  of  soils,  373. 

Mineral  matter,  dissolved  by  water,  191 ; 
in  solution,  removed  annually  from 
England  and  Wales,  194. 

Mineral  pitch,  125. 

Minerals  constituting  rocks,  9 ;  list  of,  11. 

Mineral  variation  of  rocks,  cause  of,  9. 

Mineral  wax,  128. 

Minette,  74. 

Minnesota,  wind  action  in,  297. 


Mississippi,  flood  plain  of,  323. 

Mississippi  River,  amount  of  material 
transported  by,  288. 

Missouri  River,  muddy  character  of, 
288. 

Mode  of  occurrence  of  rocks,  49. 

Monazite  sands,  origin  of,  266. 

Monoclinic  feldspars,  14. 

Monoclinic  pyroxenes  as  rock  constitu- 
ents, 21. 

Monzonite,  74. 

Moraine  defined,  55. 

Moraines,  classified,  355 ;  glacial,  290. 

Mosses,  action,  201. 

Muck,  149. 

Muscovite,  as  a  rock  constituent,  23 ;  rel- 
ative durability  of,  236. 

Natural  gas,  121. 

Nepheline  as  a  rock  constituent,  18. 

Nepheline  basalt,  107. 

Nepheline  dolerite,  104. 

Nepheline  rocks  described,  103. 

Nepheline  syenites,  78;  weathering  of, 
249. 

Nephelinite,  104. 

Nevadite,  72. 

Niggerheads,  how  formed,  244. 

Nile  delta,  section  of,  321. 

Nineveh,  site  obscured  by  sand  dunes, 
295. 

Nitrates,  influence  of,  in  feldspathic  de- 
composition; 239;  in  soils,  372;  source 
of,  372. 

Nitric  acid,  in  atmosphere,  177 ;  influence 
of,  in  feldspathic  decomposition,  239. 

Nitrogen,  in  atmosphere,  176 ;  in  soils, 372. 

Non-coking  coal,  150. 

Norites,  86. 

Noumseite,  formation  of,  226. 

Novaculite,  111. 

Nummulitic  limestone,  143. 

Obsidian,  72. 

Oldest  known  rocks,  49. 

Oligoclase,  as  a  rock  constituent,  16 ; 
disintegration  of,  241 ;  decomposition 
of,  237. 

Olivine,  as  a  rock  constituent,  24 ;  altera- 
tion into  serpentine,  24 ;  relative  dura- 
bility of,  235. 

Onyx  marbles,  113. 

Oolites,  English,  coloration  of,  258. 

Oolitic  limestone,  143 ;  origin  of,  53,  112. 

Ophicalcite,  163. 

Ophiolite,  89,  116,  163. 

Organic  acids,  action  of,  189;  corrosive 
power  on  marble,  190;  solvent  power 
augmented  by  nitrogen,  190. 


INDEX 


407 


Oriental  alabaster,  113. 

Original  constituents  of  rocks,  10. 

Original  structures  preserved  during  de- 
composition, 264. 

Orthoclase,  relative  durability  of,  236. 

Orthoclase  porphyries,  75. 

Orthoclase  as  a  rock  constituent,  14. 

Orthophyr,  76. 

Orthorhombic  pyroxenes  as  rock  constit- 
uents, 22. 

Osars,  290,  356. 

Ouachitite,  79. 

Overwash  plains,  356. 

Oxidation,  how  manifested,  187;  inci- 
dental to  decomposition,  234. 

Oxides,  silica,  10!) . 

Oxygen,  as  a  constituent  of  the  earth's 
crust,  5 ;  influence  in  preventing  loss 
of  iron  during  rock  decomposition,  239 ; 
of  the  atmosphere  as  an  agent  of  de- 
composition, 180. 

Ozokerite,  128. 

Palagonite  tuff,  140. 

Paludal  deposits,  336. 

Pantellerite,  72. 

Paraffin,  native,  128. 

Paraniorphic  minerals,  156. 

Peat,  148. 

Peat  bogs,  317. 

Pebble,  normal  shape  of,  348. 

Pegmatite,  67. 

Pelites,  135. 

Peperino,  140. 

Peridotite,  described,  95;  weathering  of , 

225. 

Peridotite-limburgite  group,  95. 
Perlite,  77. 
Perlitic  structure,  35. 
Petroleum  described,  122. 
Petrosilex,  70. 
Phenocrysts  defined,  41. 
Phlogopite,  23. 

Phonolite,  weathering  of,  217. 
Phosphates,  119. 

Phosphates  of  Tennessee,  origin  of,  267. 
Phosphatic  sandstone,  152. 
Phosphorite,  119. 
Phosphorus,    as     a    constituent   of    the 

earth's  crust,  7 ;  relative  proportion  of, 

in  rocks,  8. 
Phyllite,  169. 
Physical  and  chemical  properties  of 

rocks,  33. 

Physical  condition  of  soils,  378. 
Physical  manifestations  of  weathering, 

241. 

Picrite,  97. 
Picrite  porphyrites  described,  98. 


Picrolite,  116. 

Pic  Pourri,  decomposition  of,  by  bacteria, 

203. 

Piedmontite,  25. 
Pike's    Peak,   Colorado,  weathering    of 

granite,  243,  255. 
Pisolitic  limestone,  143. 
Pitchstone,  77. 

Placer  deposits,  origin  of,  267. 
Plagioclase  feldspars,  relative  durability 

of,  236. 

Plagioclases  as  rock  constituents,  16. 
Plant  and  animal  life,  effect  on  soils, 

389. 

Plant  life,  effect  on  soils,  394. 
Plants  and  animals,  agents  of  disintegra- 
tion, 201. 
Plutonic    rocks,    characteristics   of,  60; 

defined,  60. 

P  or  ft  do  rosso  antico,  83. 
Porphyrites,  83. 
Porphyritic  structure,  35. 
Porphyroid,  167. 

Post-Cretaceous  decay  of  granite,  272. 
Post-Glacial  decay  of  diabase,  273. 
Post-Jurassic     weathering      of      grano- 

diorites,  274. 
Post-Pliocene    weathering  of  andesites, 

274. 
Potash,  in  soils,  replacing  power  of,  370; 

soluble  in  soils,  376. 
Potassium,  as  a  constituent  of  the  earth's 

crust,    6;    relative    proportion    of,   in 

rocks,  6. 

Pot-holes,  formation  of,  196. 
Potomac  flats,  323. 
Potomac  formation,  313. 
Potstone,  101. 
Precious  serpentine,  115. 
Pre-Paloeozoic  decay  of  rocks,  275. 
Primary  rocks,  51. 
Primary  constituents  of  rocks,  10. 
Principles  involved  in   rock-weathering, 

173. 

Propyllite,  85. 
Protective  action,  of  plants,  202  ;  of  soil, 

271. 

Protogine,  67. 
Psammites,  the,  131. 
Pseudotuffs,  140. 
Psilomelane,  107. 
Puddingstone,  133. 
Pulaskite,  79. 

Pyrite,  as  a  rock  constituent,  29 ;  decom- 
position of,  29. 
Pyroclastic  rocks,  140. 
Pyrolusite,  107. 
Pyrophyllite,  116. 
Pyrophyllite  schist,  168. 


408 


INDEX 


Pyroxenes,  alteration  and  decomposition 
of,  22 ;  as  rock  constituents,  21. 

Pyroxenite-augitite  group,  99. 

Pyroxenites,  described,  99;  weathering 
of,  225. 

Quarrying  by  aid  of  fire,  in  India,  182. 

Quarry  water,  199,  254. 

Quartz,  110;  as  a  rock  constituent,  12; 

the  most  refractory  mineral,  234. 
Quartz  basalt,  92. 
Quartz-free  porphyries,  75. 
Quartz  porphyry  described,  69. 
Quartz  veins,  influence  of  contours,  260. 
Quartzite,  origin    of,   158 ;    feldspathic, 

disintegration    of,    251;     polished    by 

wind-blown  sand,  257. 
Quartzites,  weathering  of,  in  the  District 

of  Columbia,  251. 

Quaternary  deposits,  weathering  of,  258. 
Quitman  Mountains,  exfoliation  of  rocks, 

182. 

Rainfall,  amount  reaching  the  soil,  281. 

Rain  waters,  temperatures  of,  193. 

Rapilli,  140. 

Rate  of  weathering  influenced  by  texture, 
268 ;  by  composition,  269 ;  by  humidity, 
270;  by  climate,  278;  by  position,  270. 

Reaction  rims,  240. 

Regional  metamorphism,  155. 

Regolith,  classification  of,  300 ;  origin  of 
name,  299. 

Regur  defined,  382. 

Relationship  between  plutonic  and  effu- 
sive rocks,  63. 

Relative  amount  of  material  lost  through 
weathering,  284. 

Relative  durability  of  minerals,  234. 

Relative  rapidity  of  weathering  among 
eruptive  and  sedimentary  rocks,  271. 

Rensselaerite,  116. 

Residual  clays,  302;  in  caves,  233. 

Residuary  deposits,  301 ;  analysis  of,  306 ; 
names  proposed  for,  301. 

Results,  incidental  to  weathering,  266; 
of  weathering  due  to  position,  252. 

Retinite,  70,  129. 

Retinolite,  116. 

Rhodochrosite,  114. 

Rhombporphyry,  76. 

Rhyolite,  72 ;  weathering  of,  255. 

Ribbons  in  slates,  155. 

River  channels  formed  by  rock-weather- 
ing, 243. 

River  erosion,  196. 

Rivers,  flood  plains  of,  289. 

Rock,  definition  of,  1 ;  disintegration  of 
on  Bering  Island,  199. 


Rock-forming  minerals,  classification,  10 ; 
list  of,  11. 

Rocking  stones,  252. 

Rock  temperatures,  in  Africa,  183;  at 
Edinburgh,  Scotland,  184. 

Rock  -  weathering,  206  ;  a  superficial 
phenomenon,  193;  complexity  of  pro- 
cess, 240;  early  references  to,  175;  on 
Lone  Mountain,  Montana,  243. 

Rocks,  absorptive  power  of,  198 ;  chemi- 
cal composition  of,  44;  classification 
of,  57 ;  color  of,  45 ;  composed  mainly 
of  inorganic  material,  131;  composed 
of  debris  from  plants  and  animals,  141 ; 
expansion  and  contraction  under  natu- 
ral temperatures,  181 ;  formed  through 
chemical  agencies,  105 ;  formed  as  sedi- 
mentary deposits,  129;  kinds  of,  56; 
mode  of  occurrence,  49;  physical  and 
chemical  properties  of,  33  ;  specific 
gravity  of,  43. 

Roofing  slate,  microstructure  of,  170. 

Root  action,  how  manifested,  202. 

Roots,  depth  of  penetration,  in  caves 
and  soils,  202. 

Rosso  de  Levante,  98. 

Rottenstone,  origin  of,  267. 

Salt,  common,  119 ;  disintegrating  effects 
of,  198. 

Sand,  seolian,  346;  Sorby's  classification 
of,  342 ;  of  dunes,  sources  of,  296. 

Sand  blast  carving,  186 ;  natural,  185. 

Sand  dunes,  346 ;  formation  of,  295 ;  rate 
of  movement,  296. 

Sand  grains,  lasting  power  of,  197. 

Sandpipes,  formation  of,  260. 

Sandstone,  cause  of  disintegration,  247; 
cementing  matter  of,  132;  induration 
of,  256;  siliceous,  weathering  of,  228; 
spheroidal,  weathering  in,  247;  un- 
equal weathering  of,  248. 

Sandstone  concretions,  formation  of,  37. 

Sandstones,  weathering  of,  249. 

Sanidin,  kaolinization  of,  238. 

Sanidin-oligoclase  trachyte,  77. 

Saprolite,  301. 

Satin  spar,  117. 

Saxonite,  97. 

Scheerite,  129. 

Schistose  structure,  34. 

Schists,  the,  168;  crystalline,  weathering 
of,  in  Brazil,  251;  of  Cape  Elizabeth, 
weathering  of,  248 ;  origin  of,  156. 

Seacoast  swamps,  336. 

Secondary  constituents  of  rocks,  10. 

Secondary  minerals,  influence  of,  249. 

Sedentary  materials,  classification  of, 
300. 


INDEX 


409 


Sedimentary  rocks,  origin  of,  52. 

Seleuite,  117. 

Seneca  oil,  123. 

Septarian  nodules,  36,  114. 

Sericite,  23. 

Serpentine,  composition,  30;  after  peri- 
dotite,  97  ;  origin  of,  115, 159;  origin  of 
name,  31 ;  Harford  County,  Maryland, 
weathering  of,  226. 

Shale,  137. 

Sheet  defined,  50. 

Shell  limestone,  143. 

Shell  marl,  146. 

Shell  sand,  143. 

Shore  ice,  transportation  by,  292. 

Siderite,  114. 

Silica,  loss  of,  how  accounted  for,  237; 
lost  during  decomposition,  234;  possi- 
bility of  combination  with  iron  during 
rock  decomposition,  239;  solubility  of, 
238. 

Silicates,  114 ;  most  refractory,  235. 

Siliceous  sinter,  109. 

Silicified  wood,  110. 

Silicon  as  a  constituent  of  the  earth's 
crust,  5. 

Sill  defined,  50. 

Simplification  of  compounds  incidental 
to  weathering,  265. 

Singing  sands,  143. 

Sink-holes,  formation  of,  259. 

Slaggy  structure,  34. 

Slates,  137. 

Slaty  cleavage,  origin  of,  155. 

Slickensides  defined,  54. 

Snow,  effect  in  promoting  decomposition, 
280. 

Snowfall,  influence  compared  with  rain- 
fall, 280. 

Soapstone,  Amherst  County,  Virginia, 
weathering  of,  226;  Fairfax  County, 
Virginia,  weathering  of,  227 ;  origin  of, 
101. 

Sodium  as  a  constituent  of  the  earth's 
crust,  7. 

Sodium  chloride,  influence  in  decompos- 
ing feldspars,  178. 

Sodium  salts  in  soils,  371. 

Soil,  chemical  nature  of,  358 ;  capacity 
for  water,  379;  definition,  3:  mineral 
nature  of,  373  ;  nitrates  in,  322  ;  nitro- 
gen in,  372;  soluble  matter  of,  365; 
water  content  of,  281. 

Soil  cap,  creeping  of,  287. 

Soil  particles,  movements  of,  287. 

Soil  temperatures  at  Orono,  Maine,  184. 

Soils,  age  of,  386 ;  affected  by  plant  and 
animal  life,  389;  affected  by  winds, 
2%;  as  affected  by  man,  396;  classifi- 


cation, 381;  color  of,  384;  destructive 
process  of  formation,  360;  essential 
constituents  of,  362;  fertility  of,  361; 
fertility  dependent  on  physical  condi- 
tion, 379;  how  affected  by  climates, 
367  ;  how  affected  by  leaching,  368  ;  in- 
herited characteristics,  303,  360,  387: 
mineral  composition  of,  373;  of  arid 
regions,  character  of,  368 ;  of  arid  re- 
gions, composition  of,  369;  of  humid 
regions,  composition  of,  379;  of  Nile 
valley,  cause  of  fertility  of,  325 ;  phys- 
ical condition  of,  378 ;  resemblance  to 
parent  rock,  360;  soluble  salts  in,  369; 
the,  357  ;  weight  of,  381. 

Soluble  matter  in  fresh  and  decomposed 
rocks,  377. 

Soluble  salts  in  soils,  369. 

Solution,  189 ;  rate  increased  by  commi- 
nution, 192;  relative  amount  of  mate- 
rial removed  in,  258. 

Sounding  sand,  143. 

South  Dakota,  rock-weathering  in,  279. 

Specific  gravity  of  rocks,  43. 

Specular  iron  ore,  28. 

Sphserosiderite,  114. 

Sphagnous  mosses,  rate  of  growth,  317. 

Spheroidal  structure,  247. 

Spheroidal  weathering  of  sandstone,  247. 

Spherulitic  structure,  35. 

Spilite,  90. 

Stalactite,  113. 

Stalagmite,  113. 

Stamford  dike,  pre-Pala3ozoic  decay  of, 
275. 

Steatite,  116. 

Stone  implements,  weathered,  273. 

Stone  Mountain,  Georgia,  weathering  of, 
245. 

Stratification  defined,  53. 

Stratified  rocks,  weathering  of,  248. 

Stratified  structure,  34. 

Structure,  as  affecting  weathering,  249 ; 
of  rocks,  33. 

Sub-soil  defined,  383. 

Succinite  described,  128. 

Sulphates,  117. 

Sulphuric  acid  formed  during  rock- 
weathering,  205. 

Swamp  deposits,  section  of,  317. 

Swamp  soils,  315. 

Swam  ps.causeof,  316  Classification  of, 317. 

Syenite,  Little  Rock,  Arkansas,  weather- 
ing of,  214. 

Syenite-trachyte  group,  73. 

Syenites  described,  73. 

Table  Mountain  structure,  how  produced, 
252. 


410 


INDEX 


Tachylite,  92. 

Talus,  defined,  54;  slopes,  319. 

Temperatures,  effect  on  soils,  367. 

Tephrite  and  basauite  described,  94. 

Terminal  moraines,  355. 

Termites,  effects  on  soils,  391. 

Termites,  or  white  ants,  as  promoters  of 

decomposition,  204. 
Terra  rossa,  302. 
Teschenite,  89. 
Theralite-basanite  group,  93. 
Thin  sections,  preparation  of,  42. 
Till,  138. 

Time  considerations,  268. 
Time  limit  of  decay,  272. 
Titanic  iron  as  a  rock  constituent,  28. 
loadstone,  70. 
Tonalite,  82. 
Trachytes  described,  76. 
Transportation  and  deposition  of  debris, 

286. 
Transported  materials,  classification  of, 

318. 

Trap  rocks,  89. 
Trass,  140. 
Travertine,  113. 
Trees,  effect  on  soils,  394. 
Triassic    conglomerate,    weathering    of, 

264. 

Trichites  defined,  41. 
Triclinic  feldspars,  15. 
Tri  polite,  142. 
Trowlesworthite,  68. 
Tufa,  112. 
Tuffoids,  140. 
Tuffs,  139. 

Uintaite  described,  127. 
Ulmic  acid,  189. 
Unakite,  68. 

Valley  drift,  356. 

Valleys,    formed    by    decomposition    of 

greenstone  dikes,  244. 
Valleys  of  solution,  253. 
Variolite,  89,  90. 
Vegetable    matter,  decomposing    action 

of,  203. 

Veins  defined,  54. 
Verd  antique,  116. 
Verde  di  Genora,  98,  205. 
Verde  di  Pegli,  98. 
Verde  di  Prato,  205. 
Vesicular  structure,  34. 
Viridite,  30. 

Vitreous  or  glassy  structure,  33. 
Vitrophyr,  70. 
Vogesite,  74. 
Volcanic  ashes,  140. 


Volcanic  dust,  140,  298,  349. 

Volcanic    group    of    fragmeutal    rocks, 

139. 

Volcanic  mud,  140. 
Volcanic  neck  defined,  51. 
Volcanic  necks,  origin  of,  261. 

Wacke,  139,  311. 

Wad,  107. 

Water,  action  of,  in  dry  soil,  379 ;  amount 
absorbed  by  rocks,  198;  apparent  pro- 
tective action  of,  253 ;  chemical  action 
of,  186;  contents  of  soil,  281;  effects 
of  freezing,  199;  expansive  force  of 
freezing,  198;  in  cavities  of  quartz, 
199 ;  mechanical  action  of,  195 ;  solvent 
power  augmented,  186 ;  solvent  power 
tested,  191. 

Water  and  ice,  influence  in  transporting 
rock  debris,  287. 

Wave  erosion,  rapidity  of,  198. 

Waves,  erosive  action  of,  198. 

Weathering,  character  of,  indicative  of 
climate,  284;  defined,  174;  difference 
in  kind  in  cold  and  warm  climates,  283 ; 
effacement  of  characteristics  of,  262; 
incidental  results,  266;  influenced  by 
crystalline  structure,  243;  influenced 
by  mineral  composition,  248 ;  influ- 
enced by  position,  270;  influenced  by 
structure  of  rock  masses,  244 ;  irregu- 
lar, due  -to  lack  of  homogeneity,  251; 
of  andesites,  274;  of  argillite,  Har- 
ford  County,  Maryland,  229 ;  of  basalt, 
Bohemia,  223;  of  basalt,  France,  223; 
of  calcareous  rocks  containing  silicate 
minerals,  249;  of  chert,  230;  of  clastic 
rocks,  228;  of  crystalline  schists,  251; 
of  diabase,  Medford,  Massachusetts, 
218;  of  diabase,  Venezuela,  222;  of 
diabase,  Stamford,  Connecticut,  275; 
of  diorite,  Albemarle  County,  Virginia, 
224;  of  dolomitic  limestones,  250;  of 
eruptive  and  sedimentary  rocks,  rela- 
tive rapidity  of,  271 ;  of  feldspathic 
quartzite,  251 ;  of  fine-grained  homo- 
geneous rocks,  250;  of  gneiss,  Albe- 
marle County,  Virginia,  213 ;  of  granite 
of  the  District  of  Columbia,  206;  of 
granite,  Lake  Huron,  275;  of  granite, 
Pike's  Peak,  243;  of  grano-diorites, 
274;  of  limestone,  232,  250;  of  lime- 
stones, process  one  of  solution,  231 ;  of 
peridotite,  225;  of  phonolite,  217;  of 
pyroxenite,  225;  of  quartzite  boulders 
on  deserts,  256 ;  of  quartzite  in  the  Dis- 
trict of  Columbia,  251;  of  rhyolite, 
255;  of  soapstone,  Albemarle  County, 
Virginia,  226;  of  soapstone,  Fairfax 


INDEX 


411 


County,  Virginia,  227 ;  of  syenite,  Lit- 
tle Rock,  Arkansas,  214;  rate  of,  2(58; 
rate  of,  influenced  by  climate,  278 ;  rela- 
tive amount  of  material  lost  through, 
284 ;  surface  contours  due  to,  257 ;  ulti- 
mate product  of,  388;  unequal,  of 
bedded  rocks,  253. 

Weathered  stone  implements,  273. 

Websterite,  100. 

Wehrlite,  97. 

Weight  of  soils,  381. 

Whirlwinds,  effects  of,  346. 

White  ants,  effects  on  soils,  391. 

Williamsite,  115. 

Wind  action,  153,  184,  292. 

Wind  action  on  Cape  Cod,  297. 


Wind  action  on  Wyoming  soils,  296. 
Wind-blown  sand  polish,  257. 
Wisconsin,  rock-weathering  in,  278. 
Wurtzilite  described,  126. 

Zeolites,  as  conservators  of  potash,  374; 
as  rock  constituents,  31 ;  at  Plombieres, 
375 ;  composition  of,  32 ;  formed  in  deep- 
sea  bottoms,  375;  in  soils,  374;  origin 
of,  31 ;  products  of  hydro-metamor- 
phism,  375. 

Zeolitic  matter  in  soils,  370. 

Zircon  syenite,  79. 

Zonal  structure,  37. 

Zonal  structure  incident  to  weathering, 
258. 


ELEMENTARY 
PHYSICAL    GEOGRAPHY, 

By  RALPH  S.  TARR,  B.S.,  F.G.S.A., 

Assistant  Professor  of  Dynamic  Geology  and  Physical  Geography  at 

Cornell  University  : 
Author  of  "Economic  Geology  of  the  United  States." 

8vo.     Cloth.     488  pp.     Price  $1.40,  net. 


Comments. 

"  I  regard  Professor  Tarr's  book  as  one  of  the  first  publications  in  this 
country  to  embody  the  new  principles  and  advanced  methods  in  the 
study  of  physical  geography.  ...  It  seems  to  me  eminently  adapted 
as  regards  its  style  and  the  nature  of  the  illustrations  for  the  grade  of 
students  for  whom  it  is  intended ;  to  wit,  those  of  high  schools.  Most 
of  the  book  is,  indeed,  written  in  a  style  so  simple  and  plain  that  par- 
ticularly the  part  of  the  work  relating  to  physical  geography  might  well 
find  a  place  in  the  upper  class  of  many  of  the  grammar  schools."  —  J.  B. 
WOODWORTH,  Instructor  in  Geology,  Harvard  University,  Cambridge, 
Mass. 

"  I  have  recommended  the  study  of  Professor  Tarr's  admirable  book 
to  be  required  of  students  entering  the  Engineering  Department  of  the 
University  of  Michigan."  —  Professor  ISRAEL  C.  RUSSELL,  Department 
of  Science,  University  of  Michigan,  Ann  Arbor,  Mich. 

"  It  is  beyond  question  the  most  thoroughly  scientific  elementary 
text-book  on  this  important  subject  which  has  yet  appeared."  —  Boston 
Daily  Advertiser. 

"  The  subject  is  treated  with  scientific  breadth,  accuracy,  and  fulness, 
and  is  presented  in  an  exceedingly  attractive  manner.  The  style  is 
clear,  forcible,  and  instructive.  In  fact,  the  entire  arrangement  of  divi- 
sions and  subdivisions  of  the  subject,  with  abundant  illustrations,  most 
aptly  and  beautifully  executed,  explanatory  of,  and  giving  increased 
interest  to,  the  text,  altogether  makes  the  work  a  valuable  contribution 
to  science  and  well  adapted  to  the  use  of  schools  and  colleges. "- 
F.  B.  WATSON,  Superintendent  of  Schools,  Chatham,  Va. 


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66   FIFTH   AVENUE,   NEW  YORK. 


PHYSICAL   GEOGRAPHY. 


Comments. 

"  I  have  received  Professor  Tarr's  Physical  Geography,  and  have  read 
it  with  very  great  pleasure.  It  gives  an  excellent  and  accurate  presen- 
tation of  the  important  facts  relative  to  the  surface  of  the  earth,  and  the 
forces  acting  upon  it."  —  DAVID  S.  JORDAN,  President  Stanford  Univer- 
sity, Cat. 

"After  a  careful  reading,  I  do  not  hesitate  to  pronounce  it  a  most 
excellent  book.  Professor  Tarr  has  given  us  a  book  that  has  long  been 
needed  in  the  preparatory  schools,  not  of  merely  one  phase  of  the  sub- 
ject, but  covering,  and  well  too,  the  entire  subject  of  physical  geog- 
raphy."— JAMES  PERRIN  SMITH,  Associate  Professor  of  Geology,  Stanford 
University,  Cal. 

"  I  have  reviewed  the  book  very  carefully,  and  it  is  excellent.  The 
chapter  on  storms  is  especially  worthy  of  commendation.  I  have  no 
hesitation  in  recommending  it  as  in  every  way  well  adapted  for  use  in 
the  class-room.  The  mechanical  execution  of  the  book  is  beautiful. 
The  list  of  reference  books  at  the  end  of  each  chapter  makes  it  espe- 
cially valuable  to  teachers  and  students."  —  EDWARD  H.  MCLACHLIN, 
Superintendent  of  Schools,  South  Hadley,  Mass. 

"  I  like  the  book  very  much.  It  is  fresh  and  modern  in  style,  and 
presents  the  subject  in  an  attractive  manner.  I  shall  recommend  its 
use  here  next  year."  —  EDWARD  M.  SHEPARD,  Department  of  Biology 
and  Geology,  Drury  College,  Springfield,  Mo. 

"  I  have  found  it  exceedingly  valuable  and  helpful.  In  clear,  orderly 
treatment,  in  the  selection,  character,  and  number  of  illustrations,  in  the 
prominence  given  to  the  physical  features  as  illustrated  in  our  own  coun- 
try, in  the  references  to  the  bibliography  of  the  various  subjects,  it  is 
certainly  very  much  the  best  book  accessible  to  the  American  teacher." 
—  CHARLES  B.  SCOTT,  State  Normal  School,  Oswego,  N.  Y. 

"  Its  simplicity  of  statement,  very  full  treatment  of  all  points  worthy 
of  consideration,  and  lavish  use  of  most  excellent  illustrations,  call  forth 
my  hearty  approval  and  admiration."  —  CHARLES  F.  KING,  Master  Dear- 
born School,  Boston  Highlands,  Mass. 


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ECONOMIC    GEOLOGY 


OF  THE 


UNITED    STATES, 

With  Briefer  Mention  of  Foreign  Mineral   Products. 
By  RALPH  S.  TARR,  B.S.,  F.G.S.A., 

Assistant  Professor  of  Geology  at  Cornell  University. 

Second  Edition,  Revised.     $3.50. 


Comments. 

"  I  am  more  than  pleased  with  your  new  '  Economic  Geology  of  the  United  States.' 
An  introduction  to  this  subject,  fully  abreast  of  its  recent  progress,  and  especially 
adapted  to  American  students  and  readers,  has  been  a  desideratum.  The  book  is 
admirably  suited  for  class  use,  and  I  shall  adopt  it  as  the  text-book  for  instruction  in 
Economic  Geology  in  Colorado  College.  It  is  essentially  accurate,  while  written  in  a 
pleasant  and  popular  style,  and  is  one  of  the  few  books  on  practical  geology  that  the 
general  public  is  sure  to  pronounce  readable.  The  large  share  of  attention  given  to 
non-metallic  resources  is  an  especially  valuable  feature."  —  FRANCIS  W.  CRAGIN, 
Professor  of  Geology,  Mineralogy,  and  Paleontology  at  Colorado  College. 

"  I  have  examined  Professor  R.  S.  Tarr's  '  Economic  Geology '  with  much  pleasure. 
It  fills  a  felt  want.  It  will  be  found  not  only  very  helpful  to  students  and  teachers  by 
furnishing  the  fundamental  facts  of  the  science,  but  it  places  within  easy  reach  of  the 
business  man,  the  capitalist,  and  the  statesman,  fresh,  reliable,  and  complete  statistics 
of  our  national  resources.  The  numerous  tables  bringing  out  in  an  analytic  way  the 
comparative  resources  and  productiveness  of  our  country  and  of  different  states,  are 
a  specially  convenient  and  admirable  feature.  The  work  is  an  interesting  demonstra- 
tion of  the  great  public  importance  of  the  science  of  geology."  —  JAMES  E.  TODD, 
State  Geologist,  South  Dakota. 

"  It  is  one  of  those  books  that  are  valuable  for  what  it  omits,  and  for  the  concise 
method  of  presenting  its  data.  The  American  engineer  has  now  the  ability  to  acquire 
the  latest  knowledge  of  the  theories,  locations,  and  statistics  of  the  leading  American 
ore  bodies  at  a  glance.  Were  my  course  one  of  text-books,  I  should  certainly  use  it, 
and  I  have  already  called  the  attention  of  my  students  to  its  value  as  a  book  of  refer- 
ence."—  EDWARD  H.WILLIAMS,  Professor  of  Mining,  Engineering,  and  Geology  at 
Lctiigh  University. 

"  I  have  taken  time  for  a  careful  examination  of  the  work,  and  it  gives  me  pleas- 
ure to  say  that  it  is  very  satisfactory.  Regarded  simply  as  a  general  treatise  on 
Economic  Geology,  it  is  a  distinct  advance  on  anything  that  we  had  before;  while 
in  its  relations  to  the  Economic  deposits  of  this  country,  it  is  almost  a  new  creation, 
and  certainly  supplies  a  want  long  and  keenly  felt  by  both  teachers  and  general 
students.  Its  appearance  was  most  timely  in  my  case,  and  my  class  in  Economic 
Geology  are  already  using  it  as  a  text-book."  —  WILLIAM  O.  CROSBY,  Assistant 
Professor  of  Structural  and  Economic  Geology  at  the  Massachusetts  Institute  of 
Technology, 

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66    FIFTH   AVENUE,    NEW  YORK. 


By  SIR   ARCHIBALD   GEIKIE. 

A  TEXT-BOOK  OF  GEOLOGY. 

Third  Edition.     $7.50. 

"  Mr.  Geikie  is  not  a  mere  man  of  learning,  but  a  very  successful  original  thinker;  he  is 
also  endowed  with  excellent  judgment  in  the  fields  in  which  his  special  work  has  not  lain ; 
and  he  possesses  in  a  high  degree  the  literary  faculty.  This  happy  union  of  qualifications 
has  enabled  him  to  write  a  text-book  which  marks  an  era  in  British  science.  .  .  .  This 
text-book,  while  stating  conclusions  as  definitely  as  the  condition  of  the  science  warrants, 
points  out  the  uncertainty  of  many  of  them,  and  is  full  of  suggestion  as  to  what  is  yet  to  be 
learned,  and  how  it  may  be  approached.  In  short,  it  is  a  candid  and  suggestive  book  as 
well  as  a  sound  one."  —  The  Nation. 

OUTLINES  OF  FIELD  GEOLOGY. 

16mo.     $1.00. 

"  The  work  is  already  favorably  known  in  this  country,  especially  as  a  hand-book  for 
teachers,  but  in  revising  it  the  author  has  so  far  extended  its  scope  and  changed  its  order 
as  to  make  it  practically  a  new  book,  far  more  perfectly  adapted  to  its  purpose  than  the 
original  pamphlet  was."  —  New  York  Evening  Post. 

GEOLOGICAL  SKETCHES, 

AT    HOME    AND    ABROAD. 
With  Illustrations.      12mo.      $1.50. 

"  Most  of  these  sketches  have  found  their  way  into  different  periodicals,  but  the  lover 
of  geology  will  be  glad  to  see  them  in  this  form.  Dr.  Geikie  adds  to  his  great  acquire- 
*ments  in  this  branch  of  science  a  remarkable  faculty  of  expressing  his  ideas  in  clear  and 
attractive  language.  The  book  does  not  pretend  to  give  an  exhaustive  geological  treat- 
ment of  any  country,  but  describes  prominent  features  of  various  lands.  .  .  .  After  read- 
ing the  book  one  has  a  keener  interest  in  the  earth,  and  the  study  of  its  structure  and  the 
changes  that  have  taken  place."  —  The  School  Journal. 

CLASS-BOOK  OF  GEOLOGY. 

Third  Edition.    Illustrated.    12mo.    $1.10. 

"  We  have  no  hesitation  in  declaring  the  book  an  excellent  one,  containing  exactly  .  ^^a 
material  as  renders  it  especially  fitted  for  instruction.  More  than  that,  to  the  per?  jn  with 
no  geological  turn  of  mind  the  whole  matter  is  so  well  combined,  and  the  explanations  so 
simple,  that  by  reading  the  volume,  nature's  actions  in  the  past,  as  in  the  present,  can  be 
better  understood."  —  New  York  Times. 

"Professor  Geikie  is  recognized  in  England  as  one  of  the  leading  authouties  in  this 
science,  and  his  interesting  work  is  unusually  well  adapted  to  create  and  fostr.-  a  taste  for 
this  subject.  While  witten  in  a  remarkably  plain  and  simple  style,  it  contains  an  immense 
amount  of  solid  and  useful  information,  and  is  profusely  illustrated  with  engravings  of  the 
various  fossils  and  geological  formations."  —  Popular  Science  News. 

ELEMENTARY  LESSONS  IN  PHYSICAL 
GEOGRAPHY. 

Illustrated.      18mo.      $1.10. 

"  The  language  is  always  simple  and  clear,  and  the  descriptions  of  the  various  ph  >nom- 
ena  are  no  less  vivid  than  interesting ;  the  lessons  are  never  dull,  never  wearisome  and 
they  can  scarcely  fail  to  make  the  study  of  Physical  Geography  popular  wherever  they  jre 
used."  —  Academy. 

Questions  on  the  Same  for  Use  in  Schools.      18mo.      40  cents. 


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